Gravitational lenses are a simple consequence of the theory of general relativity, which predicts that light rays will be deflected as the pass concentrations of mass. This implies that cosmologically distant galaxies can be multiply-imaged when they pass close to intervening galaxies and clusters of galaxies. This is called strong lensing. Even when the images are not multiplied, they are distorted in a correlated fashion. This is called weak lensing. However, gravitational lensing is more than an interesting optical effect; it is a powerful tool for learning about the universe and its contents. In particular the measurement of weak lensing provides one of the most promising approaches to determining the equation of state of dark energy and monitoring the growth of large scale structure in the distribution of dark matter, which is one of the main reasons for constructing LSST. My current interest in gravitational lensing includes, measuring the Hubble constant using measurements of the time delay between multiple images of variable sources, developing and calibrating new formalisms for describing ray propagation in an inhomogeneous universe and using microlensing of stars by planets and smaller bodies to detect or limit the population of interstellar "nomads" which could propagate microbial life throughout the Galaxy.Measurement of the Hubble constant from Suyu et al 2011.