Abstract: In many geometric situations, separation of variables renders a Dirac operator as a generalized ordinary differential equation, with some specific properties, though. In joint work with Werner Ballmann and Gilles Carron I have abstracted from this fact the notion of Dirac system which will be discussed in this lecture. Being essentially a one dimensional notion, no specific knowledge of the geometry and not even the specific definition of the Dirac operator is necessary to understand the vital properties of such systems. We will apply the theory to be developed to spectral properties of Dirac operators near infinity, in particular to the determination of the essential spectrum.