Fast radio bursts (FRBs) are bright pulses of emission at radio wavelengths (seen mostly at wavelengths of tens of centimeters) whose physical mechanism(s) are mysterious. The bursts last between hundredths of a millisecond to a few milliseconds, and none of them has been associated with a specific source, even though thousands of FRBs have been detected since the first one was spotted fourteen years ago. Equally puzzling is the fact that most FRBs do not repeat, one of the reasons why follow-up observations to identify the originating sources are so difficult. Nevertheless, a small minority of FRBs do repeat, and four of these "repeaters" have been found to originate within host galaxies whose environments include modest star formation, possibly a clue to the nature of the objects or environments responsible for them.