Abstract: Measurements of the Cosmic Microwave Background (CMB) radiation have driven our understanding of the universe and the physics that govern its evolution, from primordial quantum fluctuations at the dawn of time to its current state. They provide the foundation for the remarkable 6-parameter cosmological model, ΛCDM, which fits all cosmological data, although there are some tensions that may hint at new physics. Far from being the last word in cosmology, the model raises deep questions: Is Inflation correct? What is its energy scale? What is the dark matter? What is the nature of dark energy? What is the neutrino mass scale and are there other light relics?  This talk will review the state of the field and describe the next-generation ground-based Stage IV CMB experiment, CMB-S4, which is designed to make breakthroughs in these areas while opening up the potential for new discoveries in physics and astrophysics.