The marine environment is crucial to Earth's ecosystem, but anthropogenic and natural sources of radiation pose significant risks to both ocean ecosystems and human health. Radionuclides, while powerful tools as tracers, can be transported in seawater, and contaminate water, sediment, and marine life through various pathways including ingestion. This chapter outlines the various sampling techniques and instruments used for radiation detection in marine environments, along with their advantages and limitations. It also covers monitoring technologies, quality assurance, and quality control procedures. Radiation measurements for marine environments employ a variety of methodologies depending on the sampling method. Some methods directly detect radiation levels, while others focus on identifying the presence of long-lived radionuclides at trace and ultra-trace concentrations. Radiation detection plays a vital role in generating a baseline of radiation levels in marine environments, the impact of radioactive discharge from nuclear reactors and other industrial bodies in biota and marine ecosystems and finding the abnormal deviation in a level of radioactive substance.