Time Delay Cosmography
  Gravitational time delays, observed in strong lens systems where the variable
background source is multiply-imaged by a massive galaxy in the foreground,
provide direct measurements of cosmological distance that are very
complementary to other cosmographic probes. The success of the technique
depends on the availability and size of a suitable sample of lensed quasars or
supernovae, precise measurements of the time delays, accurate modeling of the
gravitational potential of the main deflector, and our ability to characterize
the distribution of mass along the line of sight to the source. We review the
progress made during the last 15 years, during which the first competitive
cosmological inferences with time delays were made, and look ahead to the
potential of significantly larger lens samples in the near future.
