"I propose to develop new observational probes of cosmology basedon gravitational lensing. The project consists of three themes:1) Computer tools will be developed for detecting and measuring small-scale structures inthe distribution of dark matter using strong gravitational lenses. This will resolve one of thefew persistent conflicts between observations and the predictions of the Cold Dark Matter(CDM) model. The tools will enable the scientific community to take full advantage of presentimaging data and future data coming from missions like the proposed EUCLID satellite.2) There are several proposed and ongoing surveys designed to measure the weakgravitational lensing of galaxies across large regions of the sky and to measure the redshiftsof hundreds of millions of galaxies. To maximize the use of this data I propose a joint analysisof redshift and lensing surveys that will increase the sensitivity to the expansion history of the Universe and separate two observational consequences of dark energy which are otherwise degenerate in weak lensing data alone. This would enable us to verify whether generalrelativity is valid on cosmological scales.3) The feasibility of studying cosmology using future radio observations of the 21 cm radiation coming from early epochs of the Universe will be investigated. My work has shown that thegravitational lensing of this radiation could provide powerful constraints on the density of the Universe, dark energy's equation of state, and the masses of neutrinos. It should also bepossible to actually map the distribution of dark matter across large sections of sky with highfidelity. My goal is to treat the observational noise and systematics realistically and determinewhat impact telescope design and survey strategy have on the scientific output."