diff --git a/modules/maths/euclid_gcd.f90 b/modules/maths/euclid_gcd.f90
index 0c2990f..24dba82 100644
--- a/modules/maths/euclid_gcd.f90
+++ b/modules/maths/euclid_gcd.f90
@@ -1,4 +1,12 @@
 !> Module implementing the Euclidean Algorithm for GCD
+!!
+!!  Modified by: Ramy-Badr-Ahmed (https://github.com/Ramy-Badr-Ahmed)
+!!  in Pull Request: #31
+!!  https://github.com/TheAlgorithms/Fortran/pull/31
+!!
+!!  Please mention me (@Ramy-Badr-Ahmed) in any issue or pull request
+!!  addressing bugs/corrections to this file. Thank you!
+!!
 !! Reference: https://en.wikipedia.org/wiki/Euclidean_algorithm
 
 module gcd_module
@@ -10,6 +18,10 @@ function gcd(a, b) result(val)
         integer, value :: a, b
         integer :: t, val
 
+        ! Ensure the GCD is non-negative
+        a = abs(a)
+        b = abs(b)
+
         ! Euclidean algorithm for GCD
         do while (b /= 0)
             t = b
diff --git a/modules/maths/fibonacci.f90 b/modules/maths/fibonacci.f90
index 606e483..9a36358 100644
--- a/modules/maths/fibonacci.f90
+++ b/modules/maths/fibonacci.f90
@@ -1,4 +1,12 @@
 !> Module for Fibonacci series calculations
+!!
+!!  Modified by: Ramy-Badr-Ahmed (https://github.com/Ramy-Badr-Ahmed)
+!!  in Pull Request: #31
+!!  https://github.com/TheAlgorithms/Fortran/pull/31
+!!
+!!  Please mention me (@Ramy-Badr-Ahmed) in any issue or pull request
+!!  addressing bugs/corrections to this file. Thank you!
+!!
 !! Provides both recursive and iterative implementations.
 !! Reference: https://en.wikipedia.org/wiki/Fibonacci_number
 
@@ -10,7 +18,9 @@ module fibonacci_module
     recursive function fib_rec(n) result(f)
         integer, intent(in), value :: n
         integer :: f
-        if (n <= 1) then
+        if (n < 0) then
+            f = -1  ! signal error condition
+        else if (n <= 1) then
             f = n
         else
             f = fib_rec(n - 1) + fib_rec(n - 2)
@@ -22,7 +32,9 @@ function fib_itr(n) result(f)
         integer, intent(in) :: n
         integer :: f, tmp, f_1
         integer :: i
-        if (n <= 1) then
+        if (n < 0) then
+            f = -1  ! signal error condition
+        else if (n <= 1) then
             f = n
         else
             f_1 = 0
diff --git a/tests/maths/eculid_gcd.f90 b/tests/maths/eculid_gcd.f90
new file mode 100644
index 0000000..29f034d
--- /dev/null
+++ b/tests/maths/eculid_gcd.f90
@@ -0,0 +1,94 @@
+!> Test program for the GCD module
+!!
+!!  Created by: Ramy-Badr-Ahmed (https://github.com/Ramy-Badr-Ahmed)
+!!  in Pull Request: #31
+!!  https://github.com/TheAlgorithms/Fortran/pull/31
+!!
+!!  Please mention me (@Ramy-Badr-Ahmed) in any issue or pull request
+!!  addressing bugs/corrections to this file. Thank you!
+!!
+!!  This program provides test cases to validate the GCD function in the gcd_module.
+
+program tests_gcd_module
+    use gcd_module
+    implicit none
+    integer :: result, expected
+
+    ! Run test cases
+    call test_gcd_positive_numbers()
+    call test_gcd_with_zero()
+    call test_gcd_negative_numbers()
+    call test_gcd_same_numbers()
+    call test_gcd_large_numbers()
+
+    print *, "All tests completed."
+
+contains
+
+    ! Test case 1: GCD of two positive numbers
+    subroutine test_gcd_positive_numbers()
+        integer :: a, b
+        a = 48
+        b = 18
+        expected = 6
+        result = gcd(a, b)
+        call assert_test(result, expected, "Test 1: GCD of two positive numbers")
+    end subroutine test_gcd_positive_numbers
+
+    ! Test case 2: GCD with one number as zero
+    subroutine test_gcd_with_zero()
+        integer :: a, b
+        a = 0
+        b = 5
+        expected = 5
+        result = gcd(a, b)
+        call assert_test(result, expected, "Test 2: GCD with one number as zero")
+    end subroutine test_gcd_with_zero
+
+    ! Test case 3: GCD of two negative numbers
+    subroutine test_gcd_negative_numbers()
+        integer :: a, b
+        a = -48
+        b = -18
+        expected = 6
+        result = gcd(a, b)
+        call assert_test(result, expected, "Test 3: GCD of two negative numbers")
+    end subroutine test_gcd_negative_numbers
+
+    ! Test case 4: GCD of the same number
+    subroutine test_gcd_same_numbers()
+        integer :: a, b
+        a = 42
+        b = 42
+        expected = 42
+        result = gcd(a, b)
+        call assert_test(result, expected, "Test 4: GCD of the same number")
+    end subroutine test_gcd_same_numbers
+
+    ! Test case 5: GCD of large numbers
+    subroutine test_gcd_large_numbers()
+        integer :: a, b
+        a = 123456
+        b = 789012
+        expected = 12
+        result = gcd(a, b)
+        call assert_test(result, expected, "Test 5: GCD of large numbers")
+    end subroutine test_gcd_large_numbers
+
+    !> Subroutine to assert the test results
+    subroutine assert_test(actual, expected, test_name)
+        integer, intent(in) :: actual, expected
+        character(len=*), intent(in) :: test_name
+
+        if (actual == expected) then
+            print *, test_name, " PASSED"
+        else
+            print *, test_name, " FAILED"
+            print *, "Expected: ", expected
+            print *, "Got: ", actual
+            stop 1
+        end if
+
+    end subroutine assert_test
+
+end program tests_gcd_module
diff --git a/tests/maths/factorial.f90 b/tests/maths/factorial.f90
new file mode 100644
index 0000000..1e5d497
--- /dev/null
+++ b/tests/maths/factorial.f90
@@ -0,0 +1,87 @@
+!> Test program for factorial functions
+!!
+!!  Created by: Ramy-Badr-Ahmed (https://github.com/Ramy-Badr-Ahmed)
+!!  in Pull Request: #31
+!!  https://github.com/TheAlgorithms/Fortran/pull/31
+!!
+!!  Please mention me (@Ramy-Badr-Ahmed) in any issue or pull request
+!!  addressing bugs/corrections to this file. Thank you!
+!!
+!!  This program provides test cases to validate the two facotrial functions in the factorial_module.
+
+program tests_factorial
+    use factorial_module
+    implicit none
+
+    integer :: result, expected
+
+    ! Run test cases
+    call test_factorial()
+    call test_recursive_factorial()
+
+    print *, "All tests completed."
+
+contains
+
+    ! Test case for iterative factorial for known values
+    subroutine test_factorial()
+        expected = 120
+        result = factorial(5)
+        call assert_test(result, expected, "Test 1: Iterative Factorial of 5")
+
+        expected = 1
+        result = factorial(0)
+        call assert_test(result, expected, "Test 2: Iterative Factorial of edge case: 0")
+
+        expected = 1
+        result = factorial(1)
+        call assert_test(result, expected, "Test 3: Iterative Factorial of edge case: 1")
+
+        expected = 40320
+        result = factorial(8)
+        call assert_test(result, expected, "Test 4: Iterative Factorial of 8")
+
+        expected = 720
+        result = factorial(6)
+        call assert_test(result, expected, "Test 5: Iterative Factorial of 6")
+    end subroutine test_factorial
+
+    ! Test case for recursive factorial for known values
+    subroutine test_recursive_factorial()
+        expected = 120
+        result = recursive_factorial(5)
+        call assert_test(result, expected, "Test 1: Recursive Factorial of 5")
+
+        expected = 1
+        result = recursive_factorial(0)
+        call assert_test(result, expected, "Test 2: Recursive Factorial of edge case: 0")
+
+        expected = 1
+        result = recursive_factorial(1)
+        call assert_test(result, expected, "Test 3: Recursive Factorial of edge case: 1")
+
+        expected = 40320
+        result = recursive_factorial(8)
+        call assert_test(result, expected, "Test 4: Recursive Factorial of 8")
+
+        expected = 720
+        result = recursive_factorial(6)
+        call assert_test(result, expected, "Test 5: Recursive Factorial of 6")
+    end subroutine test_recursive_factorial
+
+    !> Subroutine to assert the test results
+    subroutine assert_test(actual, expected, test_name)
+        integer, intent(in) :: actual, expected
+        character(len=*), intent(in) :: test_name
+
+        if (actual == expected) then
+            print *, test_name, " PASSED"
+        else
+            print *, test_name, " FAILED"
+            print *, "Expected: ", expected
+            print *, "Got: ", actual
+            stop 1
+        end if
+    end subroutine assert_test
+
+end program tests_factorial
diff --git a/tests/maths/fibonacci.f90 b/tests/maths/fibonacci.f90
new file mode 100644
index 0000000..bf4b086
--- /dev/null
+++ b/tests/maths/fibonacci.f90
@@ -0,0 +1,121 @@
+!> Test program for Fibonacci functions
+!!
+!!  Created by: Ramy-Badr-Ahmed (https://github.com/Ramy-Badr-Ahmed)
+!!  in Pull Request: #31
+!!  https://github.com/TheAlgorithms/Fortran/pull/31
+!!
+!!  Please mention me (@Ramy-Badr-Ahmed) in any issue or pull request
+!!  addressing bugs/corrections to this file. Thank you!
+!!
+!!  This program provides test cases to validate the two fibonacci functions in the fibonacci_module.
+
+program tests_fibonacci
+    use fibonacci_module
+    implicit none
+
+    integer :: result, expected
+
+    ! Run test cases
+    call test_fib_zero()
+    call test_fib_one()
+    call test_fib_two()
+    call test_fib_three()
+    call test_fib_five()
+    call test_fib_thirty()
+    call test_fib_negative_one()
+    call test_fib_negative_five()
+
+    print *, "All tests completed."
+
+contains
+
+    ! Test case 1: Fibonacci of 0
+    subroutine test_fib_zero()
+        expected = 0
+        result = fib_rec(0)
+        call assert_test(result, expected, "Test 1: Fibonacci of 0 (Recursive)")
+        result = fib_itr(0)
+        call assert_test(result, expected, "Test 1: Fibonacci of 0 (Iterative)")
+    end subroutine test_fib_zero
+
+    ! Test case 2: Fibonacci of 1
+    subroutine test_fib_one()
+        expected = 1
+        result = fib_rec(1)
+        call assert_test(result, expected, "Test 2: Fibonacci of 1 (Recursive)")
+        result = fib_itr(1)
+        call assert_test(result, expected, "Test 2: Fibonacci of 1 (Iterative)")
+    end subroutine test_fib_one
+
+    ! Test case 3: Fibonacci of 2
+    subroutine test_fib_two()
+        expected = 1
+        result = fib_rec(2)
+        call assert_test(result, expected, "Test 3: Fibonacci of 2 (Recursive)")
+        result = fib_itr(2)
+        call assert_test(result, expected, "Test 3: Fibonacci of 2 (Iterative)")
+    end subroutine test_fib_two
+
+    ! Test case 4: Fibonacci of 3
+    subroutine test_fib_three()
+        expected = 2
+        result = fib_rec(3)
+        call assert_test(result, expected, "Test 4: Fibonacci of 3 (Recursive)")
+        result = fib_itr(3)
+        call assert_test(result, expected, "Test 4: Fibonacci of 3 (Iterative)")
+    end subroutine test_fib_three
+
+    ! Test case 5: Fibonacci of 5
+    subroutine test_fib_five()
+        expected = 5
+        result = fib_rec(5)
+        call assert_test(result, expected, "Test 5: Fibonacci of 5 (Recursive)")
+        result = fib_itr(5)
+        call assert_test(result, expected, "Test 5: Fibonacci of 5 (Iterative)")
+    end subroutine test_fib_five
+
+    ! Test case 6: Fibonacci of 30
+    subroutine test_fib_thirty()
+        expected = 832040
+        result = fib_rec(30)
+        call assert_test(result, expected, "Test 5: Fibonacci of 30 (Recursive)")
+        result = fib_itr(30)
+        call assert_test(result, expected, "Test 5: Fibonacci of 30 (Iterative)")
+    end subroutine test_fib_thirty
+
+    ! Test case 7: Fibonacci of negative input
+    subroutine test_fib_negative_one()
+        expected = -1
+        result = fib_rec(-9)
+        call assert_test(result, expected, "Test 6: Fibonacci of -1 (Recursive)")
+        result = fib_itr(-9)
+        call assert_test(result, expected, "Test 6: Fibonacci of -1 (Iterative)")
+    end subroutine test_fib_negative_one
+
+    ! Test case 8: Fibonacci of negative input
+    subroutine test_fib_negative_five()
+        expected = -1
+        result = fib_rec(-9)
+        call assert_test(result, expected, "Test 7: Fibonacci of -5 (Recursive)")
+        result = fib_itr(-9)
+        call assert_test(result, expected, "Test 7: Fibonacci of -5 (Iterative)")
+    end subroutine test_fib_negative_five
+
+    !> Subroutine to assert the test results
+    subroutine assert_test(actual, expected, test_name)
+        integer, intent(in) :: actual, expected
+        character(len=*), intent(in) :: test_name
+
+        if (actual == expected) then
+            print *, test_name, " PASSED"
+        else
+            print *, test_name, " FAILED"
+            print *, "Expected: ", expected
+            print *, "Got: ", actual
+            stop 1
+        end if
+
+    end subroutine assert_test
+
+end program tests_fibonacci
+