An ultra-short period (USP) planet is a type of exoplanet with an orbital period of less than one Earth day. At this short distance, tidal interactions lead to relatively rapid orbital and spin evolution. Therefore when there is a USP planet around a mature main-sequence star, it is most likely that the planet has a circular orbit and is tidally locked. There are not many USP planets with sizes exceeding 2 Earth radii. There is a strong negative correlation between their occurrence rate and the mass of the host star. About one out of 200 Sun-like stars has an ultra-short-period planet, and their occurrence rate falls from 1.1%±0.4% for M dwarfs to 0.15%±0.05% for F dwarfs. Ultra-short-period planets also tend to orbit older stars. USP planets seem mostly consistent with an Earth-like composition of 70% rock and 30% iron, but K2-229b has a higher density, suggesting a more massive iron core. WASP-47e and 55 Cnc e, however, have a lower density and are compatible with pure rock, or a rocky-iron body surrounded by a layer of water.