Use the Agile Testing Quadrants as your map of the testing landscape. Use the Test Automation Pyramid as a compass to guide you in your test automation efforts.
If you are working with legacy code that has no tests, use automation through the UI to create a software vise so that you can safely restructure your code to create oases of code covered by automated unit tests.
22. Diagram Credits Lisa Crispin and Janet Gregory Agile Testing: A Practical Guide for Testers and Agile Teams Addison-Wesley Professional; January 9, 2009. Mike Cohn Succeeding with Agile: Software Development Using Scrum Addison-Wesley Professional; November 5, 2009.