diff --git a/MathPad/MPException.h b/MathPad/MPException.h
new file mode 100644
index 0000000..eee7df0
--- /dev/null
+++ b/MathPad/MPException.h
@@ -0,0 +1,15 @@
+//
+//  MPException.h
+//  MathPad
+//
+//  Created by Kim Wittenburg on 19.04.14.
+//  Copyright (c) 2014 Kim Wittenburg. All rights reserved.
+//
+
+#ifndef MathPad_MPException_h
+#define MathPad_MPException_h
+
+extern NSString *MPIllegalSymbolException;
+extern NSString *MPIllegalSymbolExceptionSymbolKey;
+
+#endif
diff --git a/MathPad/MPException.m b/MathPad/MPException.m
new file mode 100644
index 0000000..cf228b8
--- /dev/null
+++ b/MathPad/MPException.m
@@ -0,0 +1,14 @@
+//
+//  MPException.m
+//  MathPad
+//
+//  Created by Kim Wittenburg on 19.04.14.
+//  Copyright (c) 2014 Kim Wittenburg. All rights reserved.
+//
+
+#import "MPException.h"
+#import "MPException_Private.h"
+
+NSString *MPIllegalSymbolException = @"MPIllegalSymbolException";
+NSString *MPIllegalSymbolExceptionReason = @"Only NSString and MPFunction objects are valid symbols.";
+NSString *MPIllegalSymbolExceptionSymbolKey = @"MPIllegalSymbolExceptionSymbolKey";
\ No newline at end of file
diff --git a/MathPad/MPException_Private.h b/MathPad/MPException_Private.h
new file mode 100644
index 0000000..fdee303
--- /dev/null
+++ b/MathPad/MPException_Private.h
@@ -0,0 +1,14 @@
+//
+//  MPException_Private.h
+//  MathPad
+//
+//  Created by Kim Wittenburg on 19.04.14.
+//  Copyright (c) 2014 Kim Wittenburg. All rights reserved.
+//
+
+#ifndef MathPad_MPException_Private_h
+#define MathPad_MPException_Private_h
+
+extern NSString *MPIllegalSymbolExceptionReason;
+
+#endif
diff --git a/MathPad/MPExpression.m b/MathPad/MPExpression.m
new file mode 100644
index 0000000..a4befe4
--- /dev/null
+++ b/MathPad/MPExpression.m
@@ -0,0 +1,540 @@
+//
+//  MPExpression.m
+//  MathPad
+//
+//  Created by Kim Wittenburg on 17.04.14.
+//  Copyright (c) 2014 Kim Wittenburg. All rights reserved.
+//
+
+#import "MPExpression.h"
+#import "MPFunction.h"
+#import "MPException.h"
+#import "MPException_Private.h"
+
+@interface MPExpression (MPExpressionPrivate)
+
+- (NSInteger)lengthOfSymbol:(id)symbol;
+- (void)validateSymbols:(NSArray *)symbols;
+- (NSArray *)repairedSymbols:(NSArray *)symbols; // Merges subsequent strings
+- (void)repairSymbols:(NSMutableArray *)symbols;
+- (void)getSplitOffset:(out NSUInteger *)offset inSymbolAtIndex:(out NSUInteger *)symbolIndex forSplitLocation:(NSUInteger)loc;
+
+@end
+
+@implementation MPExpression {
+    @package
+    __strong NSArray *_symbols;
+    NSInteger _length;
+}
+
+#pragma mark Creation Methods
+
+- (instancetype)initWithSymbols:(NSArray *)symbols
+{
+    [self validateSymbols:symbols];
+    self = [super init];
+    if (self) {
+        symbols = [self repairedSymbols:symbols];
+        _symbols = [[NSArray alloc] initWithArray:symbols
+                                        copyItems:YES];
+    }
+    return self;
+}
+
+#pragma mark Primitive Methods
+
+- (NSUInteger)numberOfSymbols
+{
+    return [_symbols count];
+}
+
+- (id)symbolAtIndex:(NSUInteger)index
+{
+    return _symbols[index];
+}
+
+- (double)doubleValue
+{
+#warning Unimplemented Method
+    return 0;
+}
+
+#pragma mark - NSCopying
+
+- (id)copyWithZone:(NSZone *)zone
+{
+    MPExpression *copy = [[MPExpression allocWithZone:zone] initWithSymbols:_symbols];
+    return copy;
+}
+
+#pragma mark - NSMutableCopying
+
+- (id)mutableCopyWithZone:(NSZone *)zone
+{
+    // TODO: Test Copying
+    MPMutableExpression *mutableCopy = [[MPMutableExpression allocWithZone:zone] initWithSymbols:_symbols];
+    return mutableCopy;
+}
+
+#pragma mark - NSCoding
+
+- (id)initWithCoder:(NSCoder *)aDecoder
+{
+    // TODO: Test Coding
+    return [self initWithSymbols:[aDecoder decodeObject]];
+}
+
+- (void)encodeWithCoder:(NSCoder *)aCoder
+{
+    [aCoder encodeObject:_symbols];
+}
+
+@end
+
+@implementation MPExpression (MPExpressionPrivate)
+
+- (NSInteger)lengthOfSymbol:(id)symbol
+{
+    if ([symbol isKindOfClass:[NSString class]]) {
+        return [symbol length];
+    }
+    return 1;
+}
+
+// TODO: Deal with subsequent strings
+- (void)validateSymbols:(NSArray *)symbols
+{
+    for (id symbol in symbols) {
+        if (!([symbol isKindOfClass:[NSString class]]
+              || [symbol isKindOfClass:[MPFunction class]])) {
+            @throw [NSException exceptionWithName:MPIllegalSymbolException
+                                           reason:MPIllegalSymbolExceptionReason
+                                         userInfo:@{MPIllegalSymbolExceptionSymbolKey: symbol}];
+        }
+    }
+}
+
+- (NSArray *)repairedSymbols:(NSArray *)symbols
+{
+    NSMutableArray *mutableSymbols = [symbols mutableCopy];
+    [self repairSymbols:mutableSymbols];
+    return [mutableSymbols copy];
+}
+
+- (void)repairSymbols:(NSMutableArray *)symbols
+{
+    for (NSInteger index = 1; index < symbols.count; index++) {
+        id last = symbols[index-1];
+        id current = symbols[index];
+        if ([last isKindOfClass:[NSString class]] && [current isKindOfClass:[NSString class]]) {
+            NSMutableString *new = [NSMutableString stringWithFormat:@"%@%@", last, current];
+            [symbols replaceObjectAtIndex:index-1 withObject:new];
+            [symbols removeObjectAtIndex:index];
+            index--;
+        }
+    }
+}
+
+- (void)getSplitOffset:(out NSUInteger *)offset inSymbolAtIndex:(out NSUInteger *)symbolIndex forSplitLocation:(NSUInteger)loc
+{
+    NSUInteger length = 0;
+    NSUInteger index = 0;
+    NSUInteger symbolLength = 0;
+    for (id symbol in _symbols) {
+        symbolLength = [self lengthOfSymbol:symbol];
+        length += symbolLength;
+        if (length >= loc) {
+            break;
+        }
+        index++;
+    }
+    *offset = symbolLength - (length - loc);
+    *symbolIndex = index;
+}
+
+@end
+
+@implementation MPExpression (MPExpressionExtensionMethods)
+
+#pragma mark Creation Methods
+
+- (instancetype)init
+{
+    return [self initWithSymbols:@[]];
+}
+
+- (instancetype)initWithString:(NSString *)aString
+{
+    return [self initWithSymbols:@[aString]];
+}
+
+- (instancetype)initWithFunction:(MPFunction *)aFunction
+{
+    return [self initWithSymbols:@[aFunction]];
+}
+
++ (instancetype)expression
+{
+    return [[self alloc] init];
+}
+
++ (instancetype)expressionWithString:(NSString *)aString
+{
+    return [[self alloc] initWithString:aString];
+}
+
++ (instancetype)expressionWithFunction:(MPFunction *)aFunction
+{
+    return [[self alloc] initWithFunction:aFunction];
+}
+
++ (instancetype)expressionWithSymbols:(NSArray *)symbols
+{
+    return [[self alloc] initWithSymbols:symbols];
+}
+
+#pragma mark Working with Expressions
+
+- (NSUInteger)length
+{
+    if (_length == 0) {
+        for (id symbol in _symbols) {
+            _length += [self lengthOfSymbol:symbol];
+        }
+    }
+    return _length;
+}
+
+- (MPExpression *)subexpressionFromIndex:(NSUInteger)from
+{
+    return [self subexpressionWithRange:NSMakeRange(from, [self length] - from)];
+}
+
+- (MPExpression *)subexpressionToIndex:(NSUInteger)to
+{
+    return [self subexpressionWithRange:NSMakeRange(0, to)];
+}
+
+- (MPExpression *)subexpressionWithRange:(NSRange)range
+{
+    if (NSMaxRange(range) > self.length) {
+        @throw [NSException exceptionWithName:NSRangeException reason:@"Range outside bounds of expression." userInfo:nil];
+    }
+    if (range.location == self.length || NSMaxRange(range) == 0 || range.length == 0) {
+        // Speed this up
+        return [[MPExpression alloc] init];
+    }
+    NSUInteger startOffset;
+    NSUInteger startSymbolIndex;
+    [self getSplitOffset:&startOffset inSymbolAtIndex:&startSymbolIndex forSplitLocation:range.location];
+    id startSymbol = _symbols[startSymbolIndex];
+    if (startOffset == [self lengthOfSymbol:startSymbol]) {
+        startOffset = 0;
+        startSymbolIndex++;
+        startSymbol = _symbols[startSymbolIndex];
+    } else if ([startSymbol isKindOfClass:[NSString class]]) {
+        startSymbol = [startSymbol substringFromIndex:startOffset];
+    }
+    NSUInteger endOffset;
+    NSUInteger endSymbolIndex;
+    [self getSplitOffset:&endOffset inSymbolAtIndex:&endSymbolIndex forSplitLocation:NSMaxRange(range)];
+    id endSymbol = _symbols[endSymbolIndex];
+    if ([endSymbol isKindOfClass:[NSString class]]) {
+        endSymbol = [endSymbol substringToIndex:endOffset];
+        
+    }
+    
+    NSMutableArray *symbols = [[NSMutableArray alloc] initWithCapacity:endSymbolIndex-startSymbolIndex+1];
+    [symbols addObject:startSymbol];
+    if (endSymbolIndex > startSymbolIndex + 1) {
+        NSInteger restLength = endSymbolIndex - startSymbolIndex - 1;
+        [symbols addObjectsFromArray:[_symbols subarrayWithRange:NSMakeRange(startSymbolIndex+1, restLength)]];
+    }
+    if (endSymbolIndex > startSymbolIndex) {
+        [symbols addObject:endSymbol];
+    } else if (endSymbolIndex == startSymbolIndex && [startSymbol isKindOfClass:[NSString class]]) {
+        NSString *result = [_symbols[startSymbolIndex] substringWithRange:NSMakeRange(startOffset, endOffset-startOffset)];
+        [symbols replaceObjectAtIndex:0 withObject:result];
+    }
+    return [[MPExpression alloc] initWithSymbols:symbols];
+}
+
+- (BOOL)isEqual:(id)object
+{
+    if (self == object) {
+        return YES;
+    }
+    if (object == nil) {
+        return NO;
+    }
+    if (![object isKindOfClass:[MPExpression class]]) {
+        return NO;
+    }
+    return [self isEqualToExpression:(MPExpression *)object];
+}
+
+- (BOOL)isEqualToExpression:(MPExpression *)anExpression
+{
+    // TODO: Use ->_symbols or .symbols
+    return [_symbols isEqualToArray:anExpression->_symbols];
+}
+
+// TODO: prefix and suffix operators
+
+- (MPExpression *)expressionByAppendingString:(NSString *)aString
+{
+    return [self expressionByAppendingSymbols:@[aString]];
+}
+
+- (MPExpression *)expressionByAppendingFunction:(MPFunction *)aFunction
+{
+    return [self expressionByAppendingSymbols:@[aFunction]];
+}
+
+- (MPExpression *)expressionByAppendingExpression:(MPExpression *)anExpression
+{
+    return [self expressionByAppendingSymbols:anExpression.symbols];
+}
+
+- (MPExpression *)expressionByAppendingSymbols:(NSArray *)symbols
+{
+    return [[MPExpression alloc] initWithSymbols:[_symbols arrayByAddingObjectsFromArray:symbols]];
+}
+
+#pragma mark Evaluating Expressions
+
+- (float)floatValue
+{
+    return (float)[self doubleValue];
+}
+
+- (int)intValue
+{
+    return (int)[self doubleValue];
+}
+
+- (NSInteger)integerValue
+{
+    return (NSInteger)[self doubleValue];
+}
+
+- (long long)longLongValue
+{
+    return (long long)[self doubleValue];
+}
+
+#pragma mark Querying an Expression's Contents
+
+- (NSArray *)symbols
+{
+    // _symbols is immutable so it is ok to just return it instead of making a copy
+    return _symbols;
+}
+
+- (NSString *)description
+{
+#warning Unimplemented Method
+    /*
+     Join the symbols regarding pre- and suffix operators. If there is no operator a * should be inserted
+     */
+    return [super description];
+}
+
+- (NSUInteger)hash
+{
+    return [_symbols hash];
+}
+
+@end
+
+@interface MPMutableExpression (MPMutableExpressionPrivate)
+
+- (NSArray *)makeSymbols:(NSArray *)symbols mutable:(BOOL)mutable copySymbols:(BOOL)copy;
+
+@end
+
+@implementation MPMutableExpression (MPMutableExpressionPrivate)
+
+- (NSArray *)makeSymbols:(NSArray *)symbols mutable:(BOOL)mutable copySymbols:(BOOL)copy
+{
+    NSMutableArray *newSymbols = [[NSMutableArray alloc] initWithCapacity:symbols.count];
+    for (id symbol in symbols) {
+        id newSymbol;
+        if ([symbol isKindOfClass:[NSString class]]) {
+            if (mutable) {
+                newSymbol = [symbol mutableCopy];
+            } else {
+                newSymbol = [symbol copy];
+            }
+        } else {
+            if (copy) {
+                newSymbol = [symbol copy];
+            } else {
+                newSymbol = symbol;
+            }
+        }
+        [newSymbols addObject:newSymbol];
+    }
+    return newSymbols;
+}
+
+@end
+
+// A mutable expression uses NSMutableString objects instead of NSString objects
+@implementation MPMutableExpression
+
+- (instancetype)initWithSymbols:(NSArray *)symbols
+{
+    [self validateSymbols:symbols];
+    self = [super initWithSymbols:nil];
+    if (self) {
+        symbols = [self repairedSymbols:symbols];
+        _symbols = [self makeSymbols:symbols
+                             mutable:YES
+                         copySymbols:YES];
+    }
+    return self;
+}
+
+- (id)symbolAtIndex:(NSUInteger)index
+{
+    id symbol = _symbols[index];
+    if ([symbol isKindOfClass:[NSString class]]) {
+        return [symbol copy];
+    }
+    return symbol;
+}
+
+- (NSArray *)symbols
+{
+    return [self makeSymbols:_symbols mutable:NO copySymbols:NO];
+}
+
+- (void)replaceSymbolsInRange:(NSRange)range withSymbols:(NSArray *)symbols
+{
+    if (NSMaxRange(range) > self.length) {
+        @throw [NSException exceptionWithName:NSRangeException reason:@"Range out of bounds of expression." userInfo:nil];
+    }
+    [self validateSymbols:symbols];
+    
+    // Locate the position, split the symbols
+    NSUInteger startIndex;
+    [self splitSymbolsAtLocation:range.location insertionIndex:&startIndex];
+    
+    // Perform the deletion
+    if (range.length > 0) {
+        NSUInteger endIndex;
+        [self splitSymbolsAtLocation:NSMaxRange(range) insertionIndex:&endIndex];
+        NSMutableIndexSet *indexes = [[NSMutableIndexSet alloc] init];
+        for (NSUInteger index = startIndex; index < endIndex; index++) {
+            [indexes addIndex:index];
+        }
+        [(NSMutableArray *)_symbols removeObjectsAtIndexes:indexes];
+    }
+    
+    // Perform the insertion
+    if (symbols.count > 0) {
+        NSArray *newSymbols = [self makeSymbols:symbols mutable:YES copySymbols:YES];
+        [(NSMutableArray *)_symbols replaceObjectsInRange:NSMakeRange(startIndex, 0) withObjectsFromArray:newSymbols];
+    }
+    
+    // Revalidate structure and invalidate length
+    [self repairSymbols:(NSMutableArray *)_symbols];
+    _length = 0;
+}
+
+- (void)splitSymbolsAtLocation:(NSUInteger)loc insertionIndex:(out NSUInteger *)insertionIndex
+{
+    NSUInteger splitSymbolIndex;
+    NSUInteger splitOffset;
+    [self getSplitOffset:&splitOffset inSymbolAtIndex:&splitSymbolIndex forSplitLocation:loc];
+    id splitSymbol = _symbols[splitSymbolIndex];
+    NSInteger splitSymbolLength = [self lengthOfSymbol:splitSymbol];
+    if (splitOffset == splitSymbolLength) {
+        splitOffset = 0;
+        splitSymbolIndex++;
+    }
+    if (splitOffset != 0) {
+        NSMutableString *leftPart = [[splitSymbol substringToIndex:splitOffset] mutableCopy];
+        NSMutableString *rightPart = [[splitSymbol substringFromIndex:splitOffset] mutableCopy];
+        [(NSMutableArray *)_symbols replaceObjectAtIndex:splitSymbolIndex withObject:leftPart];
+        splitSymbolIndex++;
+        [(NSMutableArray *)_symbols insertObject:rightPart atIndex:splitSymbolIndex];
+    }
+    *insertionIndex = splitSymbolIndex;
+}
+
+@end
+
+@implementation MPMutableExpression (MPMutableExpressionExtensionMethods)
+
+- (void)insertString:(NSString *)aString atIndex:(NSUInteger)loc
+{
+    [self insertSymbols:@[aString] atIndex:loc];
+}
+
+- (void)insertFunction:(MPFunction *)aFunction atIndex:(NSUInteger)loc
+{
+    [self insertSymbols:@[aFunction] atIndex:loc];
+}
+
+- (void)insertExpression:(MPExpression *)anExpression atIndex:(NSUInteger)loc
+{
+    [self insertSymbols:anExpression.symbols atIndex:loc];
+}
+
+- (void)insertSymbols:(NSArray *)symbols atIndex:(NSUInteger)loc
+{
+    [self replaceSymbolsInRange:NSMakeRange(loc, 0)
+                    withSymbols:symbols];
+}
+
+- (void)deleteSymbolsInRange:(NSRange)range
+{
+    [self replaceSymbolsInRange:range
+                    withSymbols:@[]];
+}
+
+- (void)appendString:(NSString *)aString
+{
+    [self appendSymbols:@[aString]];
+}
+
+- (void)appendFunction:(MPFunction *)aFunction
+{
+    [self appendSymbols:@[aFunction]];
+}
+
+- (void)appendExpression:(MPExpression *)anExpression
+{
+    [self appendSymbols:anExpression.symbols];
+}
+
+- (void)appendSymbols:(NSArray *)symbols
+{
+    [self replaceSymbolsInRange:NSMakeRange(self.length, 0)
+                    withSymbols:symbols];
+}
+
+- (void)setString:(NSString *)aString
+{
+    [self setSymbols:@[aString]];
+}
+
+- (void)setFunction:(MPFunction *)aFunction
+{
+    [self setSymbols:@[aFunction]];
+}
+
+- (void)setExpression:(MPExpression *)anExpression
+{
+    [self setSymbols:anExpression.symbols];
+}
+
+- (void)setSymbols:(NSArray *)symbols
+{
+    [self replaceSymbolsInRange:NSMakeRange(0, self.length)
+                    withSymbols:symbols];
+}
+
+@end
\ No newline at end of file
diff --git a/MathPad/MPFunction.m b/MathPad/MPFunction.m
new file mode 100644
index 0000000..5424a0c
--- /dev/null
+++ b/MathPad/MPFunction.m
@@ -0,0 +1,135 @@
+//
+//  MPFunction.m
+//  MathPad
+//
+//  Created by Kim Wittenburg on 18.04.14.
+//  Copyright (c) 2014 Kim Wittenburg. All rights reserved.
+//
+
+#import "MPFunction.h"
+
+@implementation MPFunction
+
+#pragma mark Creation Methods
+
+- (instancetype)init
+{
+    self = [super init];
+    if (self) {
+    }
+    return self;
+}
+
+#pragma mark Children
+
+- (NSUInteger)numberOfChildren
+{
+    return 0;
+}
+
+- (MPExpression *)childAtIndex:(NSUInteger)index
+{
+    return nil;
+}
+
+- (void)setChild:(MPExpression *)child atIndex:(NSUInteger)index
+{}
+
+#pragma mark Evaluating Functions
+
+- (double)doubleValue
+{
+    return 0;
+}
+
+#pragma mark Working With Functions
+
+- (BOOL)isEqual:(id)object
+{
+    if (self == object) {
+        return YES;
+    }
+    if (object == nil) {
+        return NO;
+    }
+    if (![object isKindOfClass:[MPFunction class]]) {
+        return NO;
+    }
+    return [self isEqualToFunction:(MPFunction *)object];
+}
+
+- (BOOL)isEqualToFunction:(MPFunction *)aFunction
+{
+    return [aFunction isMemberOfClass:[MPFunction class]] && [self isMemberOfClass:[MPFunction class]];
+}
+
+#pragma mark - NSCopying
+
+- (id)copyWithZone:(NSZone *)zone
+{
+    return [[MPFunction allocWithZone:zone] init];
+}
+
+#pragma mark - NSCoding
+
+- (id)initWithCoder:(NSCoder *)aDecoder
+{
+    self = [super init];
+    if (self) {
+        
+    }
+    return self;
+}
+
+- (void)encodeWithCoder:(NSCoder *)aCoder
+{}
+
+@end
+
+@implementation MPFunction (MPFunctionExtensionMethods)
+
+#pragma mark Children
+
+- (NSArray *)children
+{
+    NSUInteger childCount = [self numberOfChildren];
+    NSMutableArray *children = [[NSMutableArray alloc] initWithCapacity:childCount];
+    for (NSInteger i = 0; i < childCount; i++) {
+        [children addObject:[self childAtIndex:i]];
+    }
+    return [children copy];
+}
+
+#pragma mark Evaluating Functions
+
+- (float)floatValue
+{
+    return (float)[self doubleValue];
+}
+
+- (int)intValue
+{
+    return (int)[self doubleValue];
+}
+
+- (NSInteger)integerValue
+{
+    return (NSInteger)[self doubleValue];
+}
+
+- (long long)longLongValue
+{
+    return (long long)[self doubleValue];
+}
+
+- (NSString *)description
+{
+    return @"[]";
+}
+
+- (NSUInteger)hash
+{
+    return 0;
+}
+
+@end
\ No newline at end of file