diff --git a/MathPad/MPSumFunction.m b/MathPad/MPSumFunction.m
index 6b649f5..93af0e6 100644
--- a/MathPad/MPSumFunction.m
+++ b/MathPad/MPSumFunction.m
@@ -9,6 +9,8 @@
 #import "MPSumFunction.h"
 #import "MPExpression.h"
 
+#import "MPExpressionEvaluator.h"
+
 @implementation MPSumFunction
 
 #pragma mark Creation Methods
@@ -97,10 +99,30 @@
 }
 
 #pragma mark Evaluating Functions
-- (double)evaluateFunction:(NSError *__autoreleasing *)error
+- (NSDecimalNumber *)evaluate:(MPParseError *__autoreleasing *)error
 {
-#warning Unimplemented Method
-    return 0.0;
+    MPExpressionEvaluator *startEvaluator = self.startExpression.evaluator;
+    NSDecimalNumber *start = [startEvaluator evaluateVariableDefinition:YES error:error];
+    if (!start) {
+        return nil;
+    }
+    NSDecimalNumber *target = [self.targetExpression evaluateWithError:error];
+    if (!target) {
+        return nil;
+    }
+    
+    MPExpressionEvaluator *sumEvaluator = self.sumExpression.evaluator;
+    NSDecimalNumber *value = [NSDecimalNumber zero];
+    for (NSDecimalNumber *iterator = start; [iterator compare:target] <= 0; iterator = [iterator decimalNumberByAdding:[[NSDecimalNumber alloc] initWithInteger:1]]) {
+        [sumEvaluator bindValue:iterator toVariableName:startEvaluator.definedVariable];
+        NSDecimalNumber *summand = [sumEvaluator evaluateWithError:error];
+        if (summand) {
+            value = [value decimalNumberByAdding:summand];
+        } else {
+            return nil;
+        }
+    }
+    return value;
 }
 
 #pragma mark Working With Functions