The cosmic microwave background (CMB) provides an unparalleled opportunity to advance our understanding of the fundamental physics of the universe. Recent and ongoing experiments have contributed to our understanding of neutrinos, dark energy, and dark matter through measurements of large-scale structure imprinted on the CMB and constrained the conditions in the early universe, tightly restricting inflationary and other cosmological models through measurements of CMB polarization. Next-generation CMB experiments like CMB-S4 will further constrain the sum of the neutrino masses and the number of relativistic species, expand our understanding of dark energy and dark matter, and set new constraints on cosmological models describing the first moments of the universe. The polarization in the CMB is faint, so future experiments must be at least an order of magnitude more sensitive than current experiments. These unprecedented levels of sensitivity require improved systematic mitigation via modeling and novel calibration techniques. I will give an overview of the science achievable with these next-generation experiments and the advances in technology that are critical for its this leap in performance.