  Dark matter particles may interact with other dark matter particles via a new
force mediated by a dark photon, $A^{\prime}$, which would be the dark-sector
analog to the ordinary photon of electromagnetism. The dark photon can obtain a
highly suppressed mixing-induced coupling to the electromagnetic current,
providing a portal through which dark photons can interact with ordinary
matter. This review focuses on $A^{\prime}$ scenarios that are potentially
accessible to accelerator-based experiments. We summarize the existing
constraints placed by such experiments on dark photons, highlight what could be
observed in the near future, and discuss the major experimental challenges that
must be overcome to improve sensitivities.
