diff --git a/ShittyLINQ/Sum.cs b/ShittyLINQ/Sum.cs
new file mode 100644
index 0000000..7ccfa92
--- /dev/null
+++ b/ShittyLINQ/Sum.cs
@@ -0,0 +1,110 @@
+using System;
+using System.Collections.Generic;
+
+namespace ShittyLINQ
+{
+    public static partial class Extensions
+    {
+        public static int Sum(this IEnumerable<int> self)
+        {
+            if (self == null)
+            {
+                throw new ArgumentNullException();
+            }
+
+            var sum = 0;
+
+            foreach (var element in self)
+            {
+                sum += element;
+            }
+
+            return sum;
+        }
+
+        public static double Sum(this IEnumerable<double> self)
+        {
+            if (self == null)
+            {
+                throw new ArgumentNullException();
+            }
+
+            var sum = 0d;
+
+            foreach (var element in self)
+            {
+                sum += element;
+            }
+
+            return sum;
+        }
+
+        public static float Sum(this IEnumerable<float> self)
+        {
+            if (self == null)
+            {
+                throw new ArgumentNullException();
+            }
+
+            var sum = 0f;
+
+            foreach (var element in self)
+            {
+                sum += element;
+            }
+
+            return sum;
+        }
+
+        public static int Sum<T>(this IEnumerable<T> self, Func<T, int?> selector)
+        {
+            if (self == null || selector == null)
+            {
+                throw new ArgumentNullException();
+            }
+
+            var sum = 0;
+
+            foreach (var element in self)
+            {
+                sum += selector(element).GetValueOrDefault();
+            }
+
+            return sum;
+        }
+
+        public static double Sum<T>(this IEnumerable<T> self, Func<T, double?> selector)
+        {
+            if (self == null || selector == null)
+            {
+                throw new ArgumentNullException();
+            }
+
+            var sum = 0d;
+
+            foreach (var element in self)
+            {
+                sum += selector(element).GetValueOrDefault();
+            }
+
+            return sum;
+        }
+
+        public static float Sum<T>(this IEnumerable<T> self, Func<T, float?> selector)
+        {
+            if (self == null || selector == null)
+            {
+                throw new ArgumentNullException();
+            }
+
+            var sum = 0f;
+
+            foreach (var element in self)
+            {
+                sum += selector(element).GetValueOrDefault();
+            }
+
+            return sum;
+        }
+    }
+}
\ No newline at end of file
diff --git a/ShittyLinqTests/SumTests.cs b/ShittyLinqTests/SumTests.cs
new file mode 100644
index 0000000..de21aa1
--- /dev/null
+++ b/ShittyLinqTests/SumTests.cs
@@ -0,0 +1,105 @@
+using Microsoft.VisualStudio.TestTools.UnitTesting;
+using ShittyLINQ;
+using System.Collections.Generic;
+
+namespace ShittyTests
+{
+    [TestClass]
+    public class SumTests
+    {
+        [DataRow(new int[] { 1, 2, 3, 4, 5 }, 15)]
+        [DataRow(new int[] { 100, 2, 3, 4, 5 }, 114)]
+        [DataRow(new int[] { 0, 0, 0 }, 0)]
+        [DataRow(new int[] { -1, -2, -3 }, -6)]
+        [DataTestMethod]
+        public void Sum_IntNumbers(IEnumerable<int> testData, int expectedResult)
+        {
+            var result = testData.Sum();
+
+            Assert.AreEqual(expectedResult, result);
+        }
+
+        [DataRow(new int[] { 1, 2, 3, 4, 5 }, 15)]
+        [DataRow(new int[] { 100, 2, 3, 4, 5 }, 114)]
+        [DataRow(new int[] { 0, 0, 0 }, 0)]
+        [DataRow(new int[] { -1, -2, -3 }, -6)]
+        [DataTestMethod]
+        public void Sum_IntNumbers_WithSelector(IEnumerable<int> testData, int expectedResult)
+        {
+            var result = testData.Sum(x => x);
+
+            Assert.AreEqual(expectedResult, result, 0.02);
+        }
+
+        [DataRow(new double[] { 1.2, 2.5, 3.6, 4.8, 5.5 }, 17.6)]
+        [DataRow(new double[] { 100.223, 2.523, 3.696, 4.8345, 5.5435 }, 116.82)]
+        [DataRow(new double[] { 0, 0, 0 }, 0)]
+        [DataRow(new double[] { -1, -2, -3 }, -6)]
+        [DataTestMethod]
+        public void Sum_DoubleNumbers(IEnumerable<double> testData, double expectedResult)
+        {
+            var result = testData.Sum();
+
+            Assert.AreEqual(expectedResult, result, 0.02);
+        }
+
+        [DataRow(new double[] { 1.2, 2.5, 3.6, 4.8, 5.5 }, 17.6)]
+        [DataRow(new double[] { 100.223, 2.523, 3.696, 4.8345, 5.5435 }, 116.82)]
+        [DataRow(new double[] { 0, 0, 0 }, 0)]
+        [DataRow(new double[] { -1, -2, -3 }, -6)]
+        [DataTestMethod]
+        public void Sum_DoubleNumbers_WithSelector(IEnumerable<double> testData, double expectedResult)
+        {
+            var result = testData.Sum(x => x);
+
+            Assert.AreEqual(expectedResult, result, 0.02);
+        }
+
+        [DataRow(new double[] { 1.2, 2.5, 3.6, 4.8, 5.5 }, 15)]
+        [DataRow(new double[] { 100.223, 2.523, 3.696, 4.8345, 5.5435 }, 114)]
+        [DataRow(new double[] { 0, 0, 0 }, 0)]
+        [DataRow(new double[] { -1, -2, -3 }, -6)]
+        [DataTestMethod]
+        public void Sum_DoubleNumbers_WithSelectorToInt(IEnumerable<double> testData, int expectedResult)
+        {
+            var result = testData.Sum(x => (int)x);
+
+            Assert.AreEqual(expectedResult, result);
+        }
+
+        [DataRow(new float[] { 1.2f, 2.5f, 3.6f, 4.8f, 5.5f }, 17.6f)]
+        [DataRow(new float[] { 100.223f, 2.523f, 3.696f, 4.8345f, 5.5435f }, 116.82f)]
+        [DataRow(new float[] { 0, 0, 0 }, 0)]
+        [DataRow(new float[] { -1, -2, -3 }, -6)]
+        [DataTestMethod]
+        public void Sum_FloatNumbers(IEnumerable<float> testData, float expectedResult)
+        {
+            var result = testData.Sum();
+
+            Assert.AreEqual(expectedResult, result, 0.02f);
+        }
+
+        [DataRow(new float[] { 1.2f, 2.5f, 3.6f, 4.8f, 5.5f }, 17.6f)]
+        [DataRow(new float[] { 100.223f, 2.523f, 3.696f, 4.8345f, 5.5435f }, 116.82f)]
+        [DataRow(new float[] { 0f, 0f, 0f }, 0f)]
+        [DataTestMethod]
+        public void Sum_FloatNumbers_WithSelector(IEnumerable<float> testData, float expectedResult)
+        {
+            var result = testData.Sum(x => x);
+
+            Assert.AreEqual(expectedResult, result, 0.02);
+        }
+
+        [DataRow(new float[] { 1.2f, 2.5f, 3.6f, 4.8f, 5.5f }, 15)]
+        [DataRow(new float[] { 100.223f, 2.523f, 3.696f, 4.8345f, 5.5435f }, 114)]
+        [DataRow(new float[] { 0f, 0f, 0f }, 0)]
+        [DataTestMethod]
+        public void Sum_FloatNumbers_WithSelectorToInt(IEnumerable<float> testData, int expectedResult)
+        {
+            var result = testData.Sum(x => (int)x);
+
+            Assert.AreEqual(expectedResult, result);
+        }
+
+    }
+}
\ No newline at end of file