Modern Cosmology, Third Edition, provides a detailed introduction to the field of cosmology. Beginning with the smooth, homogeneous universe described by a Friedmann-Lemaître-Robertson-Walker metric, this trusted resource includes careful treatments of dark energy, big bang nucleosynthesis, recombination, and dark matter. The reader is then introduced to perturbations about an FLRW universe: their evolution with the Einstein-Boltzmann equations, their primordial generation by inflation, and their observational consequences: the acoustic peaks in the CMB; the E/B decomposition in polarization; gravitational lensing of the CMB and large-scale structure; and the BAO standard ruler and redshift-space distortions in galaxy clustering. This revised third edition includes updates such as new sections on gravitational waves, line intensity mapping, and emergent analysis techniques; expanded sections of CMB secondaries; and revised figures and pedagogy. These revisions serve to enhance a comprehensive foundational text, as well as provide users with improvements that are aligned with recent advances in the field, as well as modern focuses in the classroom. Modern Cosmology, Third Edition, builds on how modern observations are rapidly revolutionizing our understanding of the universe, and supplies readers with all the tools needed to effectively work in cosmology.