During the last 25 years, downhole measurements have been increasingly used for scientific applications in marine geology and geophysics, particularly in deep-sea drilling operations. Used mostly by the oil industry to map promising formations for exploration and production of hydrocarbons, a variety of instruments have been developed that can be lowered down drill holes to extract information about the subsurface geology. In the last decade, advances in computers, software, and data transmission have greatly increased the amount and quality of data that such instruments can provide. Relatively new instruments that image the borehole wall with high resolution can reveal layers and faults that previously could be seen only in core sections. Downhole measurements play a crucial role in linking core data with regional geophysical surveys and in providing data where core sections could not be obtained. Examples of recent scientific applications and approaches are presented that address previous problems with data quality and changes in properties over time after a hole is drilled. The role of downhole measurements is discussed for two broad areas of research: the structure and composition of the Earth's crust, most of which is formed at mid-ocean ridges, and past changes in Earth's environment recorded in the deep-sea sediments overlying the crust. Finally, new emerging technologies and experiments that promise significant advantages over current methods for downhole measurements in marine geology and geophysics are discussed.¿ 1997 American Geophysical Union