The spins of stellar-mass black holes measured with gravitational waves*

With more than one hundred compact binary mergers detected in gravitational-wave data, it is becoming possible to measure the properties of the astrophysical population -- or populations -- that generate the sources. In this talk I will focus on gravitational-wave measurements of black hole spin magnitude and orientation, which can help differentiate and characterize the formation channel(s) of LIGO-Virgo-KAGRA's compact binaries. First, I will explain why these measurements are proving more challenging than what some of the early studies had forecast. Then I will discuss the existing constraints, including possible hints of correlation between spin magnitude and other parameters. Finally, I will show how these uncertain measurements will evolve as more sources are detected.