diff --git a/src/listwiz/inspect.py b/src/listwiz/inspect.py
index b7ab1c4..1f58475 100644
--- a/src/listwiz/inspect.py
+++ b/src/listwiz/inspect.py
@@ -33,5 +33,5 @@ def longest_decreasing_streak(l):
 
 
 def most_common_element(l):
-    # Find the mmost common element in the list
+    # Find the most common element in the list
     raise NotImplementedError
diff --git a/src/listwiz/replace.py b/src/listwiz/replace.py
new file mode 100644
index 0000000..f55cf3f
--- /dev/null
+++ b/src/listwiz/replace.py
@@ -0,0 +1,39 @@
+
+def replaced_element(l, element, replacement):
+    raise NotImplementedError
+
+
+def replace_sublist(l, sublist, replacement):
+    """Naive function to replace a sublist with a replacement list.
+
+    Parameters
+    ----------
+    l : list
+        List to replace all occurances of sublist.
+    sublist : list
+        A (shorter) list to search for.
+    replacement : list
+        A new list to insert instead of sublist.
+
+    Examples
+    --------
+    >>> from listwiz.replace import replace_sublist
+
+    Replace any occurance of ``[2, 3]`` with ``[3, 2]``
+
+    >>> replace_sublist([1, 2, 3, 4], [2, 3], [3, 2])
+    [1, 3, 2, 4]
+
+    """
+    len_sublist = len(sublist)
+    result = l[:]  # copy the input list
+
+    i = 0
+    while i < len(result):
+        if result[i:i+len_sublist] == sublist:
+            result[i:i+len_sublist] = replacement
+            i += len_sublist
+        else:
+            i += 1
+
+    return result
diff --git a/src/listwiz/sorting.py b/src/listwiz/sorting.py
index a9060fb..d5d910c 100644
--- a/src/listwiz/sorting.py
+++ b/src/listwiz/sorting.py
@@ -1,7 +1,6 @@
 
 def merge_sort(l):
-    """Use the merge sort algorithm to sort the list
-    elements
+    """Use the merge sort algorithm to sort the list elements.
 
     Parameters
     ----------
@@ -12,7 +11,30 @@ def merge_sort(l):
     -------
     sorted_list
     """
-    raise NotImplementedError
+    # If there is a single item, the list is already sorted, return.
+    if len(l) == 1:
+        return l
+
+    split = len(l) // 2
+    # Split and sort each part by calling merge_sort recursively
+    part1 = merge_sort(l[:split])
+    part2 = merge_sort(l[split:])
+
+    # The result is a new list adding always the smaller items from each list
+    res = []
+    while len(part1) and len(part2):
+        if part1[0] <= part2[0]:
+            res.append(part1.pop(0))
+        else:
+            res.append(part2.pop(0))
+
+    # One of the lists is not fully consumed, add all remaining elements:
+    if len(part1):
+        res.extend(part1)
+    else:
+        res.extend(part2)
+
+    return res
 
 
 def bubble_sort(l):
diff --git a/tests/test_replace.py b/tests/test_replace.py
new file mode 100644
index 0000000..9a4d6f5
--- /dev/null
+++ b/tests/test_replace.py
@@ -0,0 +1,24 @@
+import pytest
+
+from listwiz import replace as lwr
+
+
+def test_replace_element():
+    # Stub to be replaced with actual tests when implementing the function
+    pass
+
+
+def test_replace_sublist_simple():
+    # Replace sublist does not have any test right now.
+    pass
+
+
+def test_replace_sublist_overlap():
+    # The sublist may match with overlap (e.g. if it is just [1, 1])
+    # We should add one or more test that check we replace the first occurance.
+    pass
+
+
+def test_replace_sublist_empty():
+    # This is a stub for testing the bug with empty sublists.
+    pass
diff --git a/tests/test_sorting.py b/tests/test_sorting.py
index 6513813..a94e83f 100644
--- a/tests/test_sorting.py
+++ b/tests/test_sorting.py
@@ -3,18 +3,28 @@
 from listwiz import sorting as lws
 
 
-@pytest.mark.xfail(reaso="mergesort doesn't work yet!")
 def test_mergesort():
-   l = [3, 2, 1, 5, 4, 6]
-   res = lws.merge_sort(l)
-   assert res == [1, 2, 3, 4, 5, 6]
+    # Test even sized list:
+    l = [3, 2, 1, 5, 4, 6]
+    res = lws.merge_sort(l)
+    assert res == [1, 2, 3, 4, 5, 6]
+
+    # Test odd sized list:
+    l = [5, 4, 3, 2, 1]
+    res = lws.merge_sort(l)
+    assert res == [1, 2, 3, 4, 5]
+
+
+def test_mergesort_empty():
+    # Stub for a test with empty lists
+    pass
 
 
-@pytest.mark.skip(reason="Just a stub, not implemented yet")
 def test_bubble_sort():
+    # Stub for basic bubble sort tests
     pass
 
 
-@pytest.mark.skip(reason="Just a stub, not implemented yet")
 def test_selection_sort():
+    # Stub for basic selection sort tets
     pass