Explore the fascinating world of ultralight boson clouds around black holes in this lecture by Nils Siemonsen from Princeton University. Delve into an intriguing potential source of gravitational waves as part of the ICTS Summer School on Gravitational-Wave Astronomy. Learn about exotic phenomena that could be detected in upcoming observing runs of LIGO, Virgo, and KAGRA. Gain insights into the physics of these theoretical structures and their implications for gravitational wave astronomy. Understand how this cutting-edge research fits into the broader context of continuous gravitational wave sources, alongside rapidly spinning neutron stars and other astrophysical phenomena.