Pulsar wind nebulae (PWNe) were the first objects where non-thermal polarized synchrotron emission was detected. They are one of the best astrophysical labs available for the study of high-energy processes like particle acceleration, properties of relativistic outflows, and non-thermal emission. Their broad-band spectrum makes them a suitable target for many instruments, and to date they are the only objects for which there is clear and undisputed evidence for high-energy X-ray polarized emission. In recent years a canonical model has been established which has proved incredibly successful in explaining many of the observed features. All of this makes PWNe a prime candidate for any future X-ray polarimetry study. I will review here the current MHD model, what we know from polarization in the optical and radio band, and what we might learn from next-generation polarimetry.
Pulsar wind nebulae (PWNe) are bubbles of relativistic particles and magnetic field created when the ultra-relativistic wind from a pulsar interacts with the ambient medium, either SNR or ISM. The prototype, and the best studied of this entire class of objects, is the Crab Nebula. The canonical model of PWNe was first presented by Rees & Gunn, developed by Kennel & Coroniti, and is based on a relativistic MHD description. The pulsar wind is confined inside the SNR, and slowed down to non-relativistic speeds in a strong termination shock (TS).