Ever since the prescient work of Refsdal (1964, 1966), extragalactic astronomers have known that a determination of the time delay in the variation of a multiply imaged quasar could produce a measurement of the size and age of the universe. The first definite example of multiple imaging was discovered in the "double quasar" 0957+561A, B (Walsh, Carswell & Weymann 1979), and, as it seemed that this dream might be realized, several groups began monitoring it in the hope of carrying out this program. (1) It took 17 years for a universally accepted time delay to be measured in this system (Kundic et al. 1997), after a long and controversial series of papers on the subject (for a review, see Haarsma et al. 1997). The effort in constraining the models of the lensing mass distribution has paralleled the time delay observations, resulting in a robust model of the system (Grogin & Narayan 1996). Many other promising lenses have been discovered since, and there is now considerable optimism that Refsdal's technique, like many others discussed at this meeting, is ripe for exploitation. In this brief review, we will try to summarize recent developments in the study of gravitational lensing insofar as they are relevant to measurement of the Hubble constant. For more detail on the history and the basic theory, the reader is referred to Blandford & Narayan (1992); Schneider, Ehlers & Falco (1992); and Narayan & Bartelmann (1996). Recent observational and theoretical developments are presented in the 173rd IAU Symposium proceedings (Kochanek & Hewitt 1996).
1 In fact, they were doing this under false pretenses because the original estimate of the time delay was five times too large, due to an uncharacteristic slip in Young et al. (1981). Back.