Radioactive materials from natural, intentional, and accidental sources represent a persistent threat to marine ecosystems and human health. Once introduced into the ocean, radionuclides interact with water, sediments, and organisms, undergoing transport, transformation, and incorporation into food webs. Through bioaccumulation and biomagnification, they can disrupt biodiversity and expose human populations via seafood consumption. This chapter reviews the main sources of marine radioactivity, including nuclear tests, waste disposal, and nuclear accidents, and examines the mechanisms driving dispersion and ecological effects. Risk assessment tools and ecological protection frameworks are presented alongside current advances and challenges. Sustainable management strategies, such as monitoring, waste treatment, and bioremediation, are also discussed to mitigate long-term impacts on marine ecosystems and human health.