diff --git a/src/FSharpAux.Core/ResizeArray.fs b/src/FSharpAux.Core/ResizeArray.fs
index aeb9cae..3543e70 100644
--- a/src/FSharpAux.Core/ResizeArray.fs
+++ b/src/FSharpAux.Core/ResizeArray.fs
@@ -400,4 +400,36 @@ module ResizeArray =
             if curr > acc then
                 acc <- curr
                 accv <- currv
-        accv
\ No newline at end of file
+        accv
+
+/// <summary>Sums all the values in the <paramref name="array"/>.</summary>
+    /// <param name="array">The input ResizeArray.</param>
+    /// <returns>The resulting sum.</returns>   
+    let inline sum (array: ResizeArray< ^T>) =
+        checkNonNull "array" array
+        let mutable acc = LanguagePrimitives.GenericZero< ^T>
+        for i = 0 to array.Count - 1 do
+            acc <- Checked.(+) acc array.[i]
+        acc
+
+    /// <summary>Returns the sum of the results generated by applying the function (<paramref name="projection"/>) to each element of the <paramref name="array"/>.</summary>
+    /// <param name="projection">The function to transform the ResizeArray elements into the type to be summed.</param>
+    /// <param name="array">The input ResizeArray.</param>
+    /// <returns>The resulting sum.</returns>  
+    let inline sumBy ([<InlineIfLambda>] projection: 'T -> ^R) (array: ResizeArray<'T>) =
+        checkNonNull "array" array
+        let mutable acc = LanguagePrimitives.GenericZero< ^R>
+        for i = 0 to array.Count - 1 do
+            acc <- Checked.(+) acc (projection array.[i])
+        acc
+
+    /// <summary>Counts the number of elements in the <paramref name="array"/> satisfying the <paramref name="predicate"/>.</summary>
+    /// <param name="predicate">The function to transform the ResizeArray elements into the type to be summed.</param>
+    /// <param name="array">The input ResizeArray.</param>
+    /// <returns>Number of elements satisfying the <paramref name="predicate"/>.</returns>  
+    let countIf (predicate: 'T -> bool) (arr: ResizeArray<'T>): int =  
+        let mutable acc = 0
+        for i=0 to arr.Count - 1 do
+            if predicate arr.[i] then
+                acc <- acc + 1
+        acc
\ No newline at end of file
diff --git a/tests/FSharpAux.Tests/FSharpAux.Tests.fsproj b/tests/FSharpAux.Tests/FSharpAux.Tests.fsproj
index 6dcd99a..6c2773b 100644
--- a/tests/FSharpAux.Tests/FSharpAux.Tests.fsproj
+++ b/tests/FSharpAux.Tests/FSharpAux.Tests.fsproj
@@ -9,6 +9,7 @@
   <ItemGroup>
     <Compile Include="SeqTests.fs" />
     <Compile Include="ArrayTests.fs" />
+    <Compile Include="ResizeArrayTests.fs" />
     <Compile Include="Array2DTests.fs" />
     <Compile Include="JaggedArrayTest.fs" />
     <Compile Include="ListTests.fs" />
diff --git a/tests/FSharpAux.Tests/Main.fs b/tests/FSharpAux.Tests/Main.fs
index 3699ef4..c3c946c 100644
--- a/tests/FSharpAux.Tests/Main.fs
+++ b/tests/FSharpAux.Tests/Main.fs
@@ -8,6 +8,7 @@ let all =
         [
             SeqTests.seqTests
             ArrayTests.arrayTests
+            ResizeArrayTests.resizeArrayTests
             Array2DTests.array2dTests
             JaggedArrayTest.main
             ListTests.listTests
diff --git a/tests/FSharpAux.Tests/ResizeArrayTests.fs b/tests/FSharpAux.Tests/ResizeArrayTests.fs
new file mode 100644
index 0000000..debff3d
--- /dev/null
+++ b/tests/FSharpAux.Tests/ResizeArrayTests.fs
@@ -0,0 +1,35 @@
+module ResizeArrayTests
+
+open FSharpAux
+open Expecto
+
+let private emptyArray : ResizeArray<int> = ResizeArray()
+let private intArray = [6; 5; 2; 3; 2; 8] |> ResizeArray.ofList
+
+let resizeArrayTests =
+    testList "ResizeArrayTests" [
+        testList "ResizeArray.sum" [
+            testCase "Empty array sum is 0" (fun _ ->
+                Expect.equal (ResizeArray.sum emptyArray) 0 "ResizeArray.sum of empty array is not 0."
+            )
+            testCase "returns correct sum" (fun _ ->
+                Expect.equal (ResizeArray.sum intArray) 26 "ResizeArray.sum calculates incorrectly"
+            )
+        ]
+        testList "ResizeArray.sumBy" [
+            testCase "Empty array sumBy is 0" (fun _ ->
+                Expect.equal (emptyArray |> ResizeArray.sumBy (fun x -> x * 2)) 0 "ResizeArray.sumBy of empty array is not 0."
+            )
+            testCase "returns correct sum" (fun _ ->
+                Expect.equal (intArray |> ResizeArray.sumBy (fun x -> x * 2)) 52 "ResizeArray.sumBy calculates incorrectly"
+            )
+        ]
+        testList "ResizeArray.countIf" [
+            testCase "Empty array count is 0" (fun _ ->
+                Expect.equal (emptyArray |> ResizeArray.countIf (fun x -> x % 2 = 0)) 0 "ResizeArray.countIf of empty array is not 0."
+            )
+            testCase "returns correct count" (fun _ ->
+                Expect.equal (intArray |> ResizeArray.countIf (fun x -> x % 2 = 0)) 4 "ResizeArray.countIf calculates incorrectly"
+            )
+        ]
+    ]
\ No newline at end of file