Caustics are captivating light patterns created by materials bundling or diverting light by refraction or reflection. We know caustics as random side effects, appearing, for example, at the bottom of a swimming pool. In this paper we show that it is possible to control caustic patterns to form almost any desired shape by optimizing the geometry of the reflective or refractive surface generating the caustic. We demonstrate how this surprising result offers a new perspective on light control and the use of caustics as an inspiring architectural design element. Several produced prototypes illustrate that physical realizations of such optimized geometry are feasible.