The Philippine Archipelago comprises 10 major island areas and a great many small islands and islets in a triangular area between Formosa on the north and Borneo and Celebes, of Indonesia, on the south. The basement includes mainly volcanic and sedimentary rocks of unknown age, and probably of considerable time range, that have been intruded by ultrabasic to intermediate plutonic rocks as well as by younger volcanics. Fossiliferous Cretaceous, Eocene, and Oligocene sedimentary rocks are found, but it was not until Miocene and Pliocene time that recognizable geosynclinal conditions developed to produce the thick Upper Tertiary sequences now exposed as a result of the late Tertiary and Quaternary disturbances. Considerable geological mapping and exploratory drilling have be n done in areas of higher prospective value. However, the substantial differences between the intra-archipelagic environment and its resulting complex stratigraphy and geologic structure, which are in great contrast to the normally regular relations in intra-continental areas, have not been properly evaluated. Certain areas in the Philippines may possess reasonably attractive petroleum possibilities although exploratory efforts to date have been negative.