This paper discusses both natural and man-made radioactivity in the marine environment. Radioactivity occurs naturally in both the sea water and in the ocean sediment. Radioactivity in the sea water is fairly uniform geographically and is dominated by potassium-40, a naturally occurring isotope. Unlike sea water, sediment radiation levels vary with sediment type and location. The deposition of insoluble thorium isotopes formed by the decay of water-soluble uranium is the primary source of natural radiation in the sediment. Man-made sources of radioactivity, in descending order of importance, are: (1) the sinking of two U.S. and two Soviet nuclear submarines; (2) fallout from nuclear weapons testing; (3) dumping of primarily British and American low-level nuclear waste; and (4) dumping of radiated effluents from the British Windscale reprocessing facility and other European and Indian reprocessing facilities.