  Pulsar Wind Nebulae (PWNe) are ideal astrophysical laboratories where high
energy relativistic phenomena can be investigated. They are close, well
resolved in our observations, and the knowledge derived in their study has a
strong impact in many other fields, from AGNs to GRBs. Yet there are still
unresolved issues, that prevent us from a full clear understanding of these
objects. The lucky combination of high resolution X-ray imaging and numerical
codes to handle the outflow and dynamical properties of relativistic MHD, has
opened a new avenue of investigation that has lead to interesting progresses in
the last years. Despite all of this, we do not understand yet how particles are
accelerated, and the functioning of the pulsar wind and pulsar magnetosphere,
that power PWNe. I will review what is now commonly known as the MHD paradigm,
and in particular I will focus on various approaches that have been and are
currently used to model these systems. For each I will highlight its
advantages, limitations, and degree of applicability.
