The Stellar Populations of Resolved Galaxies

The study of galaxies outside the Milky Way using observations of resolved stellar populations has experienced extraordinary growth over the past two decades. There now exist panchromatic photometry and precise astrometry for hundreds of millions of individually resolved stars in hundreds of galaxies that span about10 9 in luminosity, about10 cubed in metallicity, and sample a wide range of environments. Here, we review advances in extragalactic astronomy enabled by resolved stellar population imaging. We summarize major observational programs and focus on four interconnected science areas: the star formation histories of galaxies, 'bursty' star formation, the stellar initial mass function, and the orbital histories of Local Group galaxies. We review the literature in each area, synthesize key findings, identify emerging trends, and discuss how new and future facilities (e.g., JWST, LSST, Euclid, Roman, CSST, ELTs, HWO) will enable further progress. Finally, we highlight shortcomings in current datasets and discuss promising directions for new areas of investigation.