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The cosmic microwave background (CMB), the faint afterglow of the Big Bang, is the oldest light we can observe in the Universe. Since its discovery by Penzias and Wilson in 1964, CMB observations have been central to the rise of modern precision cosmology. Together with other probes, they have revealed how the Universe began and evolved, what it is made of, and, at the same time, they have highlighted several fundamental questions that remain unanswered. In the coming decade, CMB observations are poised to take major steps forward. The Simons Observatory, the largest ground-based CMB experiment to date, has just begun observations with a particular focus on CMB polarization. Polarization measurements at degree angular scales will constrain primordial gravitational waves, probing cosmic inflation and quantum fluctuations of gravity. Fluctuations at arcminute scales, on the other hand, trace the growth of large-scale structure through gravitational lensing and galaxy cluster surveys, and can constrain so-called dark radiation—the relativistic degrees of freedom in the early universe. These rich data sets will drive substantial progress in our understanding of dark energy, neutrino masses, and the evolution of galaxies and clusters. In this talk, I will review the exciting scientific opportunities and experimental challenges of upcoming CMB measurements, and discuss how they may open new windows onto the physics of the early Universe.