  Based on the expected population of core collapse supernova remnants and the
huge number of detected pulsars in the Galaxy, still representing only a
fraction of the real population, pulsar wind nebulae are likely to constitute
one of the largest classes of {extended} Galactic sources in many energy bands.
For simple evolutionary reasons, the majority of the population is made of
evolved systems, whose detection and identification are complicated by their
reduced luminosity, the possible lack of X-ray emission (that fades
progressively away with the age of the pulsar), and by their modified
morphology with respect to young systems. Nevertheless they have gained renewed
attention in recent years, following the detection of misaligned X-ray tails
protruding from an increasing number of nebulae created by fast moving pulsars,
and of extended TeV halos surrounding aged systems. Both these features are
clear signs of an efficient escape of particles, with energy close to the
maximum acceleration limit of the pulsar. Here we discuss the properties of
those evolved systems and what we have understood about the process of particle
escape, and the formation of observed features.
