  Weak gravitational lensing is a unique probe of the dark side of the
universe: it provides a direct way to map the distribution of dark matter
around galaxies, clusters of galaxies and on cosmological scales. Furthermore,
the measurement of lensing induced distortions of the shapes of distant
galaxies is a powerful probe of dark energy. In this review we describe how
lensing measurements are made and interpreted. We discuss various systematic
effects that can hamper progress and how they may be overcome. We review some
of the recent results in weak lensing by galaxies, galaxy clusters and cosmic
shear and discuss the prospects for dark energy measurements from planned
surveys.
