The fossil record demonstrates that mammals re-entered the marine realm on at least seven separate occasions. Five of these clades are still extant, whereas two are extinct. This review presents a brief introduction to the phylogeny of each group of marine mammals, based on the latest studies using both morphological and molecular data. Evolutionary highlights are presented, focusing on changes affecting the sensory systems, locomotion, breathing, feeding, and reproduction in Cetacea, Sirenia, Desmostylia, and Pinnipedia. Aquatic adaptations are specifically cited, supported by data from morphological and geochemical studies. For example, analysis of oxygen isotopes incorporated into fossil tooth enamel indicates whether these mammals foraged in (and, therefore, ingested) fresh water or sea water. Comparisons between groups are made to see if there are any common patterns, particularly relating to adaptations to aquatic life. Results show that aquatic characteristics evolved in mosaic patterns and that different morphological solutions to aquatic conditions were achieved separately in each of these groups. Changes in the axial and appendicular skeleton assist with locomotion for aquatic foraging. Nostril and eye placement modifications accommodate wading versus underwater foraging needs. All groups exhibit aquatic adaptations directly related to feeding, particularly changes in the dentition and rostrum. The earliest representatives of these clades all show morphological features that indicate they were feeding while in the water, suggesting that feeding ecology is a key factor in the evolution of marine mammals.