Modern techniques allow experiments on a single-driven atom or a single system. The quantum jump approach was originally developed for the description of the temporal development of such a driven system and was later extended to more general situations like a moving particle coupled to a spatially confined laser beam or to spin-boson baths. In this chapter the underlying ideas are presented and illustrated by simple examples. Applications include the spectacular macroscopic light and dark periods in the fluorescence of a single atom, quantum counting processes and arrival times.