This course first explains the deep, beautiful, and immensely useful theory of light for illumination: the theory that lets us see what can or cannot be done in illumination optics, and that guides us towards viable solutions. Ètendue, the most mysterious quantity in illumination optics is fully explained and revealed as the most important quantity in illumination optics. Then, the course introduces “design patterns” in illumination optics: optical elements as building blocks, providing reusable solution approaches to recurring problems. These building blocks (e.g., TIR lenses, mixing rods and Köhler illumination) are explained by what they do to the light, and how to combine them. Practical examples as well as exercises with solutions for later self-study are an integral part of the course.