What spinning black holes reveal about how they formed

Ten years of gravitational-wave detections have logged over 100 binary black hole mergers, and their spins carry fingerprints of how those systems formed — whether in isolated stellar pairs, dense star clusters, or the chaotic disks around supermassive black holes. This review maps what we've learned: spins tend to be small, misaligned pairs hint at dynamical assembly, and correlations with mass and redshift are just becoming detectable. Next-generation detectors should turn these statistical whispers into clear formation histories.