Compact-binary “standard sirens” provide a direct, distance-ladder–independent measurement of luminosity distance from the gravitational-wave waveform, and when combined with a host-galaxy redshift they yield an estimate of H0 that is in principle free of many traditional systematics (Schutz 1986; Abbott et al. 2017). Early bright and dark standard-siren measurements from GW170817 and subsequent events give H0 values broadly consistent with both CMB and distance-ladder ranges but with wide posteriors that already hint at a preference around 67–73 km s?¹ Mpc?¹, making them an emerging arbiter in the Hubble tension and raising questions, within ΛCDM, about how to reconcile all three methods as siren precision improves (Abbott et al. 2017; Chen et al. 2018).