Many modifications to the mammalian bauplan associated with the obligate aquatic lives of cetaceans-fusiform bodies, flukes, flippers, and blowholes-are evident at a glance. But among the most strikingly unusual and divergent features of modern cetacean anatomy are the arrangements of their cranial bones: (1) bones that are situated at opposite ends of the skull in other mammals are positioned close together, their proximity resulting from (2) these bones extensively overlapping the bones that ordinarily would separate them. The term "telescoping" is commonly used to describe the odd anatomy of modern cetacean skulls, yet its usage and the particular skull features to which it refers vary widely. Placing the term in historical and biological context, this review offers an explicit definition of telescoping that includes the two criteria enumerated above. Defining telescoping in this way draws attention to many specific biological questions that are raised by the unusual anatomy of cetacean skulls; highlights the central role of sutures as the locus for changes in the sizes, shapes, mechanical properties, and connectivity of cranial bones; and emphasizes the importance of sutures in skull development and evolution. The unusual arrangements of cranial bones and sutures referred to as telescoping are not easily explained by what is known about cranial development in more conventional mammals. Discovering the evolutionary-developmental processes that produce the extensive overlap characteristic of cetacean telescoping will give insights into both cetacean evolution and the "rules" that more generally govern mammalian skull function, development, and evolution. Anat Rec, 302:1055-1073, 2019. © 2019 Wiley Periodicals, Inc.