Polarization measurements of rotation-powered pulsars have been a very powerful diagnostic in the radio band and promise to be at least as useful a diagnostic in the X-ray band. Since the relativistic particles that radiate pulsar high-energy emission tightly beam the radiation along the pulsar magnetic field, phase-resolved polarimetry has the potential to map the emission patterns. Fermi observations of young pulsars at gamma-ray energies have disfavored polar-cap models where emission takes place near the neutron star surface, and strongly favor outer-magnetosphere models where emission takes place close to the light cylinder or beyond. Since the different outer-magnetosphere models predict similar gamma-ray light curves, it is difficult to discriminate between them at gamma-ray energies. But X-ray polarization has the potential to provide this discrimination, since the models predict distinct polarization signatures and optical detections will only be possible for a small number of pulsars.