This book contains a complete and up-to-date review of gravitational lensing, focussing on recent progress in both observations and theory. The mapping of dark matter on galaxy and cluster sized scales, as will be seen through the future eyes of ALMA, is discussed. The lensing of quasars is presented as a method for measuring cosmological parameters and recent observations and modeling are examined. In the regime of weak gravitational lensing numerous results are now available and are compared with simulations at different length-scales. The use of weak lensing around isolated galaxies to measure the shape of galaxy halos is studied. Absorption lines of lensed quasars can be used to map the occulting systems out to a redshift of 2 and this technique is investigated. This book is designed for researchers and graduate students interested in any research area of gravitational lensing.