Ultralight boson particles, if they exist as theorized, could form clouds around rapidly rotating black holes through the phenomenon called superradiance. Such clouds are expected to emit long-lasting, quasimonochromatic gravitational waves that LIGO, Virgo, and KAGRA could detect. Searching for gravitational waves emitted by boson clouds around black holes provides a new cosmic approach to interrogating the existence of ultralight bosons that are difficult to probe with conventional lab experiments. In this talk, I will provide a theoretical overview of the phenomenon, describe the gravitational wave signatures, and discuss the astrophysical sources. I will briefly explain the constraints obtained from a search targeting a known galactic black hole, Cygnus X-1, and show future search prospects targeting other sources.