Strong Lensing and  \(H_0\)

Time delays from strong gravitational lensing provide a one-step absolute distance measurement. Thus, they measure \(H_0\) independently of all other probes. We first review the foundations and history of time-delay cosmography. Then, we illustrate the current state of the art by means of two recent case studies that have been real breakthroughs: (i) the quadruply imaged quasar lensed by a galaxy-scale deflector RXJ1131–1231, for which spatially resolved stellar kinematics is available; (ii) the multiply imaged supernova “Refsdal”, the first with measured time delays, lensed by cluster MACS1149.5 \(+\) 2223. We conclude by discussing the exciting future prospects of time-delay cosmography in the coming decade.