The existence of ontogenetic shifts in habitat by marine turtles, and of immature-dominated assemblages in “developmental habitat,” were important concepts first proposed by Archie Carr in 1956. Results of long-term, in-water capture programs in Caribbean Panama (17 yr) and Bermuda (37 yr) allow the testing and refinement of these ideas, in particular the developmental habitat hypothesis for Chelonia mydas, Eretmochelys imbricata, and Caretta caretta. A literature survey reviews worldwide studies on these species, and also incorporates Lepidochelys kempii. The studies in Panama and Bermuda reported in this paper use netting, mark/recapture, laparoscopy, and satellite telemetry to investigate size distributions, maturity status, residency, site fidelity, and developmental migrations of three species of sea turtles at three study sites. Characteristics of benthic developmental habitat of C. mydas, E. imbricata, L. kempii, and, to a lesser extent, C. caretta in the Atlantic Ocean usually include benthic feeding; exclusive or nearly exclusive occupation by immature animals; seasonal or multiyear residency and site fidelity in specific areas; developmental migration from the habitat before maturation; and high genetic diversity. Variation of these traits worldwide, contradictory evidence regarding the concept of developmental habitat, and evolution of this life stage are presented. Laparoscopic data provide information concerning the process of sexual maturation; mean size and size range are presented for three maturity stages of C. mydas from Panama and Bermuda, and for size at onset of puberty and maturity for Eretmochelys and Caretta in the West Atlantic. Nicaragua is the primary site of recovery of immature green turtles tagged in Bermuda, representing a developmental migration of at least 2800 km. To the extent that tag returns and stranding data represent good proxies for mortality, transitions between life stages appear to be periods of decreased survivorship.