diff --git a/README.md b/README.md
index ea9cc54..e502205 100644
--- a/README.md
+++ b/README.md
@@ -11,6 +11,11 @@ This module can now be installed using ``pip``.
 
     pip install markdown-include
 
+## Tests
+Use the unittest module
+```bash
+python -m unittest discover unittests/
+```
 
 ## Usage
 This module can be used in a program in the following way:
diff --git a/markdown_include/include.py b/markdown_include/include.py
index 5463cd5..4b53db5 100644
--- a/markdown_include/include.py
+++ b/markdown_include/include.py
@@ -82,7 +82,7 @@ def run(self, lines):
             for loc, line in enumerate(lines):
                 m = INC_SYNTAX.search(line)
 
-                if m:
+                while m:
                     filename = m.group(1)
                     filename = os.path.expanduser(filename)
                     if not os.path.isabs(filename):
@@ -91,7 +91,7 @@ def run(self, lines):
                         )
                     try:
                         with open(filename, 'r', encoding=self.encoding) as r:
-                            text = r.readlines()
+                            text = self.run(r.readlines())
                             
                     except Exception as e:
                         if not self.throwException:
@@ -102,7 +102,6 @@ def run(self, lines):
                         else:
                             raise e
 
-                    line_split = INC_SYNTAX.split(line)
                     if len(text) == 0:
                         text.append('')
                     for i in range(len(text)):
@@ -116,12 +115,12 @@ def run(self, lines):
                                     text[i] = '#' * self.headingOffset + text[i]
                         else:
                             text[i] = text[i].rstrip('\r\n')
-                            
-                    text[0] = line_split[0] + text[0]
-                    text[-1] = text[-1] + line_split[2]
-                    lines = lines[:loc] + text + lines[loc+1:]
-                    break
-                    
+                    text_to_insert = '\r\n'.join(text)
+                    line = line[:m.start()] + text_to_insert.strip() + line[m.end():]
+                    del lines[loc]
+                    lines[loc:loc] = line.split('\r\n')
+                    m = INC_SYNTAX.search(line)
+
                 else:
                     h = HEADING_SYNTAX.search(line)
                     if h:
diff --git a/unittests/resources/header.md b/unittests/resources/header.md
new file mode 100644
index 0000000..d5d88ac
--- /dev/null
+++ b/unittests/resources/header.md
@@ -0,0 +1,3 @@
+# This heading will be one level deeper from the previous heading
+More included file content.
+End of included content.
diff --git a/unittests/resources/simple.md b/unittests/resources/simple.md
new file mode 100644
index 0000000..9bfaa99
--- /dev/null
+++ b/unittests/resources/simple.md
@@ -0,0 +1 @@
+This is a simple template
diff --git a/unittests/resources/simple_2.md b/unittests/resources/simple_2.md
new file mode 100644
index 0000000..951e815
--- /dev/null
+++ b/unittests/resources/simple_2.md
@@ -0,0 +1 @@
+This is another simple template
diff --git a/unittests/resources/template_inside.md b/unittests/resources/template_inside.md
new file mode 100644
index 0000000..a20a14c
--- /dev/null
+++ b/unittests/resources/template_inside.md
@@ -0,0 +1,3 @@
+{!resources/simple.md!}
+
+This is a template with a template.
diff --git a/unittests/test_embedded.py b/unittests/test_embedded.py
new file mode 100644
index 0000000..a5e7d94
--- /dev/null
+++ b/unittests/test_embedded.py
@@ -0,0 +1,20 @@
+import os
+from unittest import TestCase
+
+import markdown
+
+from markdown_include.include import IncludePreprocessor, MarkdownInclude
+
+
+class TestEmbedded(TestCase):
+    def setUp(self) -> None:
+        self.maxDiff = None
+        self.markdown_include = MarkdownInclude(
+            configs={'base_path': os.path.dirname(os.path.realpath(__file__))}
+        )
+
+    def test_embedded_template(self):
+        source = "{!resources/template_inside.md!}"
+        html = markdown.markdown(source, extensions=[self.markdown_include])
+
+        self.assertEqual(html, "<p>This is a simple template</p>\n<p>This is a template with a template.</p>")
diff --git a/unittests/test_include.py b/unittests/test_include.py
new file mode 100644
index 0000000..62bb6c7
--- /dev/null
+++ b/unittests/test_include.py
@@ -0,0 +1,45 @@
+import os
+from unittest import TestCase
+
+import markdown
+
+from markdown_include.include import MarkdownInclude
+
+
+class TestInclude(TestCase):
+    def setUp(self) -> None:
+        self.maxDiff = None
+        self.markdown_include = MarkdownInclude(
+            configs={'base_path': os.path.dirname(os.path.realpath(__file__))}
+        )
+
+    def test_single_include(self):
+        source = "{!resources/simple.md!}"
+        html = markdown.markdown(source, extensions=[self.markdown_include])
+
+        self.assertEqual(html, '<p>This is a simple template</p>')
+
+    def test_double_include(self):
+        source = "{!resources/simple.md!} and {!resources/simple_2.md!}"
+        html = markdown.markdown(source, extensions=[self.markdown_include])
+
+        self.assertEqual(html, '<p>This is a simple template and This is another simple template</p>')
+
+    def test_headers(self):
+        source = "Source file\n" \
+                 "# Heading Level 1 of main file\n" \
+                 "{!resources/header.md!}\n" \
+                 "## Heading Level 2 of main file\n" \
+                 "{!resources/header.md!}"
+
+        html = markdown.markdown(source, extensions=[self.markdown_include])
+
+        self.assertEqual(html, "<p>Source file</p>\n"
+                                "<h1>Heading Level 1 of main file</h1>\n"
+                                "<h1>This heading will be one level deeper from the previous heading</h1>\n"
+                                "<p>More included file content.\n"
+                                "End of included content.</p>\n"
+                                "<h2>Heading Level 2 of main file</h2>\n"
+                                "<h1>This heading will be one level deeper from the previous heading</h1>\n"
+                                "<p>More included file content.\n"
+                                "End of included content.</p>")
diff --git a/unittests/test_include_header_indent.py b/unittests/test_include_header_indent.py
new file mode 100644
index 0000000..35f651d
--- /dev/null
+++ b/unittests/test_include_header_indent.py
@@ -0,0 +1,45 @@
+import os
+from unittest import TestCase
+
+import markdown
+
+from markdown_include.include import MarkdownInclude
+
+
+class TestInclude(TestCase):
+    def setUp(self) -> None:
+        self.maxDiff = None
+        self.markdown_include = MarkdownInclude(
+            configs={'base_path': os.path.dirname(os.path.realpath(__file__)), 'inheritHeadingDepth': True}
+        )
+
+    def test_single_include(self):
+        source = "{!resources/simple.md!}"
+        html = markdown.markdown(source, extensions=[self.markdown_include])
+
+        self.assertEqual(html, '<p>This is a simple template</p>')
+
+    def test_double_include(self):
+        source = "{!resources/simple.md!} and {!resources/simple_2.md!}"
+        html = markdown.markdown(source, extensions=[self.markdown_include])
+
+        self.assertEqual(html, '<p>This is a simple template and This is another simple template</p>')
+
+    def test_headers(self):
+        source = "Source file\n" \
+                 "# Heading Level 1 of main file\n" \
+                 "{!resources/header.md!}\n" \
+                 "## Heading Level 2 of main file\n" \
+                 "{!resources/header.md!}"
+
+        html = markdown.markdown(source, extensions=[self.markdown_include])
+
+        self.assertEqual(html, "<p>Source file</p>\n"
+                                "<h1>Heading Level 1 of main file</h1>\n"
+                                "<h2>This heading will be one level deeper from the previous heading</h2>\n"
+                                "<p>More included file content.</p>\n"
+                                "<p>End of included content.</p>\n"
+                                "<h2>Heading Level 2 of main file</h2>\n"
+                                "<h3>This heading will be one level deeper from the previous heading</h3>\n"
+                                "<p>More included file content.</p>\n"
+                                "<p>End of included content.</p>")
diff --git a/unittests/test_lines.py b/unittests/test_lines.py
new file mode 100644
index 0000000..a97f1fe
--- /dev/null
+++ b/unittests/test_lines.py
@@ -0,0 +1,26 @@
+import os
+from unittest import TestCase
+
+from markdown_include.include import IncludePreprocessor
+
+
+class TestProcessor(TestCase):
+    def setUp(self) -> None:
+        self.maxDiff = None
+        self.processor = IncludePreprocessor(None, {
+            "base_path": os.path.dirname(os.path.realpath(__file__)),
+            "encoding": "utf-8",
+            "inheritHeadingDepth": False,
+            "headingOffset": 0,
+            "throwException": False,
+        })
+
+    def test_lines(self):
+        source = ["Source file",
+                  "# Heading Level 1 of main file",
+                  "{!resources/header.md!}",
+                  "## Heading Level 2 of main file",
+                  "{!resources/header.md!}"]
+        result_lines = self.processor.run(source)
+
+        self.assertEqual(9, len(result_lines))