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.