The Caribbean region presents a wide array of geologic problems related to its plate tectonic evolution, the nature of its oceanic crust or basement, ocean and climate history, and the opening and closing of intra-Caribbean and Atlantic-to-Pacific seaways. With the exception of DSDP Site 502, the Caribbean has not been targeted by the Ocean Drilling Program or Deep Sea Drilling Project for more than two decades. A fresh impetus has now been given to Caribbean drilling by the recent discovery of a strewn field of unaltered impact glass spherules or tektites in Haiti and Mexico at the Cretaceous/Tertiary boundary, and the identification of their source in the 180 to 300 km-wide Chicxulub impact crater in the Yucatan. Leg 165 drilling will address two major themes: the nature of the Cretaceous/Tertiary boundary and the influence of tropical seas on global ocean history and climate evolution. Drilling at five primary sites will provide a unique opportunity to examine nearly 90 m.y. of Earth history, including (1) the K/T boundary impact event, mechanisms of ejecta dispersal, and environmental consequences from aerosols and fallout of ejecta; (2) catastrophic extinction events and biotic recovery; (3) the nature of climate forcing in the pre-Neogene world and tests of climate models with boundary conditions very different from those of today; (4) several episodes of moderate to extreme climatic warmth (early Late Cretaceous, early Eocene, early to mid Pliocene); (5) the evolution of tropical sea surface temperatures and changes in meridional temperature gradients; (6) changes in oceanic circulation and in sources of deep and intermediate water masses through Late Cretaceous and Cenozoic time; (7) the closing of low latitude oceanic gateways, the opening of a major gateway within the Caribbean, and the oceanic and climatic consequences of low latitude tectonics during the late Neogene; (8) tropical climate variability during the late Quaternary; and (9) nature and origin of Caribbean crust.