Pulsar wind nebulae.

Pulsar wind nebulae are the prototypical relativistic accelerators, and one of the foremost laboratory where the study of relativistic plasma physics, and non-thermal emission can be performed. The Crab Nebula, for example had been at the heart of many astrophysical discoveries, and its study has been the driver for much of our present understanding of particle acceleration. I will review here the current status of our knowledge of these systems, and how it has progressed in the past decade thanks to a combination of high-resolution observations and state-of-the-art numerical techniques, and how its has impacted the broader astrophysical community. I will then discuss the still many unknowns that remain, and the prospects for future advancements.