The origin of baleen in mysticete whales occurred sometime between 34 and 24 million years ago and represents a major macroevolutionary shift in cetacean morphology (teeth to baleen) and ecology (raptorial to filter feeding). This dramatic change in feeding strategy is explored by employing a diversity of tools: morphology, molecule and development. Adaptations for feeding in extinct toothed mysticetes provide the phylogenetic context for evaluating morphological apomorphies preserved in the skulls of stem and crown edentulous mysticetes. Molecular and morphological data also document the dramatic developmental shifts that take place in extant fetal baleen whales, in skull development, resorption of a fetal dentition and growth of baleen. This complex evolutionary transition entails multiple, integrated aspects of anatomy, genetics, development and ecology that are only beginning to be understood, and future work will further clarify the processes underlying this macroevolutionary pattern.