The image of the Cosmic Microwave Background (CMB) is deformed by the gravitational lensing of the matter in the Universe. In this talk I will show how we can reconstruct this lensing deflection field from CMB observations. I will then demonstrate how CMB lensing can put tight constraints on the content of the Universe, on the sum of the neutrino masses, and how it is a source of noise which can be removed to observe cosmic inflation. Lastly I will introduce a new CMB lensing estimator which will be key to optimally reconstruct the CMB lensing field with the next generation of experiments.