This course explains the basic principles and concepts of ellipsometry for thin film diagnostics. A primary goal of the course is to reveal the logic, structure, and methodology behind this useful branch of optical science and engineering. Anyone who wants to answer questions such as, "what can I measure with ellipsometry", "which ellipsometric technique to use?" or "how much confidence is there in the data obtained?" will benefit from taking this course. It unlocks the doors for understanding and using ellipsometry in practical situations.