In this talk I will discuss the next frontier of research on the Cosmic Microwave Background (CMB): precisely measuring the gravitational lensing of the CMB.  This CMB lensing signal encodes a wealth of statistical information about the distribution of matter in the Universe, which is sensitive to the total mass of the neutrinos, the nature of dark energy, and the particle properties of dark matter.    I will discuss recent progress using data from ACTPol, and forecasts for the upcoming AdvACT, Simons Observatory, and CMB-S4 experiments. I will also discuss a novel and powerful way to probe dark matter particle properties using very high resolution CMB lensing measurements, which can distinguish between cold dark matter and alternative dark matter models that suppress small-scale structure.