For more than 120 years, the coastal exposures of the Santa Cruz Formation have been fertile ground for recovery of vertebrates from the late Early Miocene (~18 to 16 million years ago, Ma). As long ago as the 1840s, Captain Bartholomew Sulivan collected fossils from this region and sent them to Charles Darwin, who passed them to Richard Owen. Carlos Ameghino undertook several explorations of the region starting in the late 1880s. Carlos' specimens were described by his brother Florentino, who believed that many of the species were more ancient than now understood and represented the ancestors of many Holarctic mammalian orders. Ameghino's novel claims prompted William B. Scott to organize fossil collecting expeditions in the Santa Cruz beds led by John B. Hatcher. The fossils were described in a series of exhaustive monographs with the conclusion that the fauna was much younger than Ameghino thought. Several brief expeditions took place during the twentieth century, led by researchers from different institutions. Since 2003, we have undertaken the collection of over 1600 specimens, including large series of relatively complete skeletons. In this edited volume we have gathered together a group of researchers to study the coastal Santa Cruz Formation and its associated flora and fauna to provide a paleobiological reconstruction of the Santacrucian vertebrate community and to place it in its biotic and physical environment.