diff --git a/scripts/pylib/twister/twister/testsuite.py b/scripts/pylib/twister/twister/testsuite.py
index 6f7207b914a..08984312c2b 100644
--- a/scripts/pylib/twister/twister/testsuite.py
+++ b/scripts/pylib/twister/twister/testsuite.py
@@ -183,7 +183,7 @@ def _find_regular_ztest_testcases(search_area, suite_regex_matches, is_registere
         re.MULTILINE)
 
     search_start, search_end = \
-        TestSuite._get_search_area_boundary(search_area, suite_regex_matches, is_registered_test_suite)
+        _get_search_area_boundary(search_area, suite_regex_matches, is_registered_test_suite)
     limited_search_area = search_area[search_start:search_end]
     testcase_names, warnings = \
         _find_ztest_testcases(limited_search_area, testcase_regex)
diff --git a/scripts/tests/twister/test_testinstance.py b/scripts/tests/twister/test_testinstance.py
new file mode 100644
index 00000000000..c0315dde5b4
--- /dev/null
+++ b/scripts/tests/twister/test_testinstance.py
@@ -0,0 +1,116 @@
+#!/usr/bin/env python3
+# Copyright (c) 2020 Intel Corporation
+#
+# SPDX-License-Identifier: Apache-2.0
+# pylint: disable=line-too-long
+"""
+Tests for testinstance class
+"""
+
+import os
+import sys
+import pytest
+
+ZEPHYR_BASE = os.getenv("ZEPHYR_BASE")
+sys.path.insert(0, os.path.join(ZEPHYR_BASE, "scripts/pylib/twister"))
+from twister.testinstance import TestInstance
+from twister.error import BuildError, TwisterException
+from twister.testsuite import TestSuite
+
+
+TESTDATA_1 = [
+    (False, False, "console", "na", "qemu", False, [], (False, True)),
+    (False, False, "console", "native", "qemu", False, [], (False, True)),
+    (True, False, "console", "native", "nsim", False, [], (True, False)),
+    (True, True, "console", "native", "renode", False, [], (True, False)),
+    (False, False, "sensor", "native", "", False, [], (True, False)),
+    (False, False, "sensor", "na", "", False, [], (True, False)),
+    (False, True, "sensor", "native", "", True, [], (True, False)),
+]
+@pytest.mark.parametrize("build_only, slow, harness, platform_type, platform_sim, device_testing,fixture, expected", TESTDATA_1)
+def test_check_build_or_run(class_testplan, monkeypatch, all_testsuites_dict, platforms_list, build_only, slow, harness, platform_type, platform_sim, device_testing, fixture, expected):
+    """" Test to check the conditions for build_only and run scenarios
+    Scenario 1: Test when different parameters are passed, build_only and run are set correctly
+    Scenario 2: Test if build_only is enabled when the OS is Windows"""
+
+    class_testplan.testsuites = all_testsuites_dict
+    testsuite = class_testplan.testsuites.get('scripts/tests/twister/test_data/testsuites/tests/test_a/test_a.check_1')
+
+
+    class_testplan.platforms = platforms_list
+    platform = class_testplan.get_platform("demo_board_2")
+    platform.type = platform_type
+    platform.simulation = platform_sim
+    testsuite.harness = harness
+    testsuite.build_only = build_only
+    testsuite.slow = slow
+
+    testinstance = TestInstance(testsuite, platform, class_testplan.env.outdir)
+    run = testinstance.check_runnable(slow, device_testing, fixture)
+    _, r = expected
+    assert run == r
+
+    monkeypatch.setattr("os.name", "nt")
+    run = testinstance.check_runnable()
+    assert not run
+
+TESTDATA_2 = [
+    (True, True, True, ["demo_board_2"], "native", '\nCONFIG_COVERAGE=y\nCONFIG_COVERAGE_DUMP=y\nCONFIG_ASAN=y\nCONFIG_UBSAN=y'),
+    (True, False, True, ["demo_board_2"], "native", '\nCONFIG_COVERAGE=y\nCONFIG_COVERAGE_DUMP=y\nCONFIG_ASAN=y'),
+    (False, False, True, ["demo_board_2"], 'native', '\nCONFIG_COVERAGE=y\nCONFIG_COVERAGE_DUMP=y'),
+    (True, False, True, ["demo_board_2"], 'mcu', '\nCONFIG_COVERAGE=y\nCONFIG_COVERAGE_DUMP=y'),
+    (False, False, False, ["demo_board_2"], 'native', ''),
+    (False, False, True, ['demo_board_1'], 'native', ''),
+    (True, False, False, ["demo_board_2"], 'native', '\nCONFIG_ASAN=y'),
+    (False, True, False, ["demo_board_2"], 'native', '\nCONFIG_UBSAN=y'),
+]
+
+@pytest.mark.parametrize("enable_asan, enable_ubsan, enable_coverage, coverage_platform, platform_type, expected_content", TESTDATA_2)
+def test_create_overlay(class_testplan, all_testsuites_dict, platforms_list, enable_asan, enable_ubsan, enable_coverage, coverage_platform, platform_type, expected_content):
+    """Test correct content is written to testcase_extra.conf based on if conditions
+    TO DO: Add extra_configs to the input list"""
+    class_testplan.testsuites = all_testsuites_dict
+    testcase = class_testplan.testsuites.get('scripts/tests/twister/test_data/testsuites/samples/test_app/sample_test.app')
+    class_testplan.platforms = platforms_list
+    platform = class_testplan.get_platform("demo_board_2")
+
+    testinstance = TestInstance(testcase, platform, class_testplan.env.outdir)
+    platform.type = platform_type
+    assert testinstance.create_overlay(platform, enable_asan, enable_ubsan, enable_coverage, coverage_platform) == expected_content
+
+def test_calculate_sizes(class_testplan, all_testsuites_dict, platforms_list):
+    """ Test Calculate sizes method for zephyr elf"""
+    class_testplan.testsuites = all_testsuites_dict
+    testcase = class_testplan.testsuites.get('scripts/tests/twister/test_data/testsuites/samples/test_app/sample_test.app')
+    class_testplan.platforms = platforms_list
+    platform = class_testplan.get_platform("demo_board_2")
+    testinstance = TestInstance(testcase, platform, class_testplan.env.outdir)
+
+    with pytest.raises(BuildError):
+        assert testinstance.calculate_sizes() == "Missing/multiple output ELF binary"
+
+TESTDATA_3 = [
+    (ZEPHYR_BASE + '/scripts/tests/twister/test_data/testsuites', ZEPHYR_BASE, '/scripts/tests/twister/test_data/testsuites/tests/test_a/test_a.check_1', '/scripts/tests/twister/test_data/testsuites/tests/test_a/test_a.check_1'),
+    (ZEPHYR_BASE, '.', 'test_a.check_1', 'test_a.check_1'),
+    (ZEPHYR_BASE, '/scripts/tests/twister/test_data/testsuites/test_b', 'test_b.check_1', '/scripts/tests/twister/test_data/testsuites/test_b/test_b.check_1'),
+    (os.path.join(ZEPHYR_BASE, '/scripts/tests'), '.', 'test_b.check_1', 'test_b.check_1'),
+    (os.path.join(ZEPHYR_BASE, '/scripts/tests'), '.', '.', '.'),
+    (ZEPHYR_BASE, '.', 'test_a.check_1.check_2', 'test_a.check_1.check_2'),
+]
+@pytest.mark.parametrize("testcase_root, workdir, name, expected", TESTDATA_3)
+def test_get_unique(testcase_root, workdir, name, expected):
+    '''Test to check if the unique name is given for each testcase root and workdir'''
+    unique = TestSuite(testcase_root, workdir, name)
+    assert unique.name == expected
+
+TESTDATA_4 = [
+    (ZEPHYR_BASE, '.', 'test_c', 'Tests should reference the category and subsystem with a dot as a separator.'),
+    (os.path.join(ZEPHYR_BASE, '/scripts/tests'), '.', '', 'Tests should reference the category and subsystem with a dot as a separator.'),
+]
+@pytest.mark.parametrize("testcase_root, workdir, name, exception", TESTDATA_4)
+def test_get_unique_exception(testcase_root, workdir, name, exception):
+    '''Test to check if tests reference the category and subsystem with a dot as a separator'''
+
+    with pytest.raises(TwisterException):
+        unique = TestSuite(testcase_root, workdir, name)
+        assert unique == exception
diff --git a/scripts/tests/twister/test_testplan_class.py b/scripts/tests/twister/test_testplan_class.py
index adb9c1cf236..0b3038dfeb7 100644
--- a/scripts/tests/twister/test_testplan_class.py
+++ b/scripts/tests/twister/test_testplan_class.py
@@ -13,9 +13,8 @@ import pytest
 ZEPHYR_BASE = os.getenv("ZEPHYR_BASE")
 sys.path.insert(0, os.path.join(ZEPHYR_BASE, "scripts/pylib/twister"))
 
-from twister.testplan import TestPlan, ScanPathResult
+from twister.testplan import TestPlan
 from twister.testinstance import TestInstance
-from twister.error import BuildError, TwisterException
 from twister.testsuite import TestSuite
 from twister.platform import Platform
 
@@ -256,3 +255,4 @@ def test_add_instances(test_data, class_env, all_testsuites_dict, platforms_list
 		   [platform.name + '/' + s for s in list(all_testsuites_dict.keys())]
     assert all(isinstance(n, TestInstance) for n in list(plan.instances.values()))
     assert list(plan.instances.values()) == instance_list
+
diff --git a/scripts/tests/twister/test_testsuite.py b/scripts/tests/twister/test_testsuite.py
new file mode 100644
index 00000000000..b64cbd92e64
--- /dev/null
+++ b/scripts/tests/twister/test_testsuite.py
@@ -0,0 +1,62 @@
+#!/usr/bin/env python3
+# Copyright (c) 2020 Intel Corporation
+#
+# SPDX-License-Identifier: Apache-2.0
+# pylint: disable=line-too-long
+"""
+Tests for testinstance class
+"""
+
+import os
+import sys
+import pytest
+
+ZEPHYR_BASE = os.getenv("ZEPHYR_BASE")
+sys.path.insert(0, os.path.join(ZEPHYR_BASE, "scripts/pylib/twister"))
+
+from twister.testsuite import scan_file, ScanPathResult
+
+TESTDATA_5 = [
+    ("testsuites/tests/test_ztest.c",
+     ScanPathResult(
+         warnings=None,
+         matches=['a', 'c', 'unit_a',
+                  'newline',
+                  'test_test_aa',
+                  'user', 'last'],
+         has_registered_test_suites=False,
+         has_run_registered_test_suites=False,
+         has_test_main=False,
+         ztest_suite_names = ["test_api"])),
+    ("testsuites/tests/test_a/test_ztest_error.c",
+     ScanPathResult(
+         warnings="Found a test that does not start with test_",
+         matches=['1a', '1c', '2a', '2b'],
+         has_registered_test_suites=False,
+         has_run_registered_test_suites=False,
+         has_test_main=True,
+         ztest_suite_names = ["feature1", "feature2"])),
+    ("testsuites/tests/test_a/test_ztest_error_1.c",
+     ScanPathResult(
+         warnings="found invalid #ifdef, #endif in ztest_test_suite()",
+         matches=['unit_1a', 'unit_1b', 'Unit_1c'],
+         has_registered_test_suites=False,
+         has_run_registered_test_suites=False,
+         has_test_main=False,
+         ztest_suite_names = ["feature3"])),
+    ("testsuites/tests/test_d/test_ztest_error_register_test_suite.c",
+     ScanPathResult(
+         warnings=None, matches=['unit_1a', 'unit_1b'],
+         has_registered_test_suites=True,
+         has_run_registered_test_suites=False,
+         has_test_main=False,
+         ztest_suite_names = ["feature4"])),
+]
+
+@pytest.mark.parametrize("test_file, expected", TESTDATA_5)
+def test_scan_file(test_data, test_file, class_env, expected: ScanPathResult):
+    '''Testing scan_file method with different ztest files for warnings and results'''
+
+    result: ScanPathResult = scan_file(os.path.join(test_data, test_file))
+    assert result == expected
+