Pulsar Wind Nebulae are highly intriguing astrophysical objects in many respects. They are the brightest and closest class of relativistic sources, and hence the ultimate laboratory for the physics of relativistic plasmas: several processes observed (or inferred to occur) in other classes of relativistic sources can here be studied with unique detail, like the acceleration and collimation of relativistic outflows, or the acceleration of particles at relativistic shocks... Here I review the current status of our theoretical understanding of Pulsar Wind Nebulae in light of the most recent 2D and 3D MHD modelling of these sources. I will discuss how these studies are taking us to the point when we can reliably use multi-wavelength observations of these nebulae as a diagnostics of the hidden physics of the pulsar wind and of the mechanism(s) through which particles are accelerated at the highly relativistic shock that terminates the wind. Finally I will briefly discuss recent progress in the modelling of evolved Pulsar Wind Nebulae and of the escape of particles from these systems. This effort is instrumental to credibly assess the role of Pulsar Winds as sources of cosmic ray leptons, and has recently been recognised to have important implications also on cosmic ray transport in the Galaxy. (read more)