Naturally occurring radioisotopes are ubiquitous in the Earth's crust, including in soil, rocks, and water. Unlike anthropogenic (human-made) sources, these natural radioisotopes contribute a baseline level of radiation to the marine environment. Marine organisms can be exposed to this natural radiation through various pathways, including direct uptake from seawater and ingestion of food containing radioisotopes. Specific radionuclides like U-238, Th-232, K-40, Po-210, and Pb-210 are readily incorporated by marine biota, potentially impacting their health. Ionizing radiation in the marine environment can also come from human activities. The level of radiation exposure in marine organisms depends on the combined effects of both natural and artificial radioactivity. The transport of radionuclides (including various radioisotopes) occurs primarily through interactions with sediments, rocks, and soil particles. Radionuclides, which are unstable atoms that emit radiation, can be present in various forms and have different half-lives. This chapter explore the transport processes of radioisotopes in the marine environment, focusing on their interactions with these geological substrates and the subsequent effects on marine life.