In two-dimensional systems, propagating waves or pulses are the dominant structures. These waves may be subject to instabilities, which lead to the formation of spiral-shaped wave structures. Although pattern formation in 2D is frequently studied experimentally, three-dimensional excitable systems present an even more rich diversity of patterns and their dynamics. We present an optical tomographic set-up that allows for the study of transparent excitable systems. The dynamics of three-dimensional wave structures, so-called scroll waves or scroll rings are introduced. The stability properties of single scroll waves will be investigated, and the theoretically proposed instabilities of scroll waves will be shown to indeed also occur in experiments. Similar dynamics is known also to play a decisive role in some cardiac arrhythmiae.