Pulsar Wind Nebulae (PWNe) are among the best objects where high energy relativistic
astrophysics, can be investigated. They are close, well resolved in our observation,
and the knowledge derived in their study has a strong impact in many other fields,
from AGNs to GRBs. They also behave as a probe of the interior of the surrounding
SNR, and their dynamical evolution, can be used to constrain parameters of the SN
ejecta, otherwise not accessible. Thanks to a lucky combination of high resolution
X-ray imaging (mostly thank to CHANDRA) and the coincidental development of numerical
codes to handle the outflow and dynamical properties of relativistic MHD, our
understanding of these system have greatly progressed in the last years. I will
review how a beautifully coherent picture has developed leading to a now, commonly
agreed paradigm, which has branched outside the field of SNRs themselves. I will also
present problems, and future possible developments, showing how PWNe continue to
provide us with new phenomenology, to challenge established truths.