  Pulsar Wind Nebulae (PWNe), structures powered by energetic pulsars, are
known for their detection across the entire electromagnetic spectrum, with
diverse morphologies and spectral behaviour between these bands. The temporal
evolution of the morphology and spectrum of a PWN depends strongly on the
properties of the associated neutron star, the relativistic outflow powered by
its rotational energy, and surrounding medium, and thereby can vary markedly
between objects. Due the continuous, but decreasing, injection of electrons and
positrons into the PWN by the pulsar, the brightness and spectral variation
within and amongst their wind nebulae reflect the magnetic field structure and
particle transport within the PWN. This can include complex motions such as
reverse flows or turbulence due to shock interactions and disruption to the
nebula. During the last stage of the PWN's evolution, when the neutron star
moves supersonically with respect to its environment, the escape of accelerated
particles into the surrounding medium creates an extensive halo evident in
very-high-energy gamma-rays. This chapter describes some of the identifying
characteristics and key aspects of pulsar wind nebulae through their several
evolutionary stages.
