Abstract　 　　
The origin of non-thermal radiation in powerful astrophysical sources with relativistic outflows, such as pulsar winds, microquasars, active galactic nuclei and gamma-ray bursts represents a central question in modern high energy astrophysics. In a generic model, particles are accelerated at a relativistic shock wave and then radiate in the ambient magnetic or radiation fields. Our understanding of the physics of particle acceleration at relativistic shock waves has progressed substantially in the past decade. This talk will discuss these aspects, and in particular when and how particle acceleration can take place at relativistic shock waves. In a second part, the talk will discuss some consequences with respect to the modelling of the afterglow of gamma-ray bursts and, if time permits, to the origin of very high energy cosmic rays.