  Pulsars steadily dissipate their rotational energy via relativistic winds.
Confinement of these outflows generates luminous pulsar wind nebulae, seen
across the electromagnetic spectrum in synchrotron and inverse Compton
emission, and in optical emission lines when they shock the surrounding medium.
These sources act as important probes of relativistic shocks, particle
acceleration and of interstellar gas. We review the many recent advances in the
study of pulsar wind nebulae, with particular focus on the evolutionary stages
through which these objects progress as they expand into their surroundings,
and on morphological structures within these nebulae which directly trace the
physical processes of particle acceleration and outflow. We conclude by
considering some exciting new probes of pulsar wind nebulae, including the
study of TeV gamma-ray emission from these sources, and observations of pulsar
winds in close binary systems.
