Planets with orbital periods less than a day, or ultra-short-period planets (USPs), are unique laboratories for observation and planet formation and evolution theories, but the planets’ origins remain unclear. Competing origins theories make different, testable predictions: for instance, USPs may be the remnants of tidally disrupted hot Jupiters, or they may have been brought in from farther out by orbital decay driven by tides raised on their host stars. If USPs were brought in by tidal decay and interactions with other planets, that would mean USP systems would often (perhaps always) host additional planets. The characteristics of the ultra-short-period planet population can therefore be used to directly test the predictions of competing planet formation theories. Over 60 candidate or confirmed USPs have been identified around stars with radii smaller than 0.7 solar radii (about a quarter of all known USPs). M dwarfs are also a quarter of the stars with at least one identified companion planet, while K dwarfs are somewhat overrepresented. None of the 14 systems with 3 or more planets orbits a star more massive than the Sun. The Short Period Planet Group (SuPerPiG) is working to discover and confirm USPs and to explore the implications of USP population characteristics for planet formation theories.