  Diffusive TeV gamma-ray emissions have been recently discovered extending
beyond the pulsar wind nebulae of a few middle-aged pulsars, implying that
energetic electron/positron pairs are escaping from the pulsar wind nebulae and
radiating in the ambient interstellar medium. It has been suggested that these
extended emissions constitute a distinct class of nonthermal sources, termed
"pulsar halos". In this article, I will review the research progress on pulsar
halos and discuss our current understanding on their physics, including the
multiwavelength observations, different theoretical models, as well as
implications for the origin of cosmic-ray positron excess and Galactic diffuse
gamma-ray emission.
