Chaos in periodically driven systems

We give a review of the most well-known examples of dynamical systems with chaotic dynamics. After a phenomenological introduction, a definition of chaos is ‘deduced’. One of the examples concerns the Hénon-attractor, that has only recently been characterized as the closure of a coiling curve. We will explain why it is plausible that this characterization also holds for more realistic cases and, in particular, for periodically forced system. As examples we use the driven damped pendulum and the laser with optical injection. Finally, we indicate some links between chaos and probability.