Lensing of the cosmic microwave background (CMB) by large scale structure is an important effect that modifies the power spectrum, induces non-Gaussianity, generates B-mode polarization and is sensitive to dark energy parameters and neutrino masses. With its recent detection (Smith et al. arXiv:0705.3980v1) and with upcoming higher resolution experiments, we are entering an era in which the full scientific potential of CMB Lensing can be exploited. This talk will explain the effect, how to simulate it and what the simulations together with upcoming data are likely to tell us.