The Atacama Cosmology Telescope (ACT) made measurements of the cosmic microwave background (CMB) for fifteen years, with a series of three cameras. ACT was designed to complement Planck, the latest and most sensitive CMB satellite. To compete with the powerful data set provided from a major space mission requires years of observations from bespoke arrays of thousands of detectors; for ACT these are cooled to 100 mK and deployed on the eponymous special-purpose 6 m telescope, on an arid plateau at 5190 m. Data from Planck have been compressed to a set of six parameters describing initial conditions for the evolution of the universe. Meanwhile, measurements of the present-day universe reveal multiple aspects of the state to which it has actually evolved. This before-and-after comparison is not new, but the precision of before-and-after parameter estimates is now dazzingly high, and interesting disagreements may be emerging. Their resolution could require new physics in the cosmological model, or deepen our understanding of astrophysics, or reveal new ways to use multiwavelength data to ask new questions about the universe. How ACT works and some of the ways its data have and will inform our understanding of cosmology and astrophysics will be discussed, capped off with a quick look toward future developments.