The measurement of very small amounts of radioactivity. This can be achieved by measuring radioactivity in large samples or modifying conventional radiation-detection instruments for greater detection sensitivity, or both. All approaches must take into account the characteristics of the radiations, notably the very short range and intense energy deposition of alpha particles, intermediate range and energy deposition of beta particles, and high penetration of matter with low linear energy transfer by gamma rays. These techniques are important for measuring radionuclides from naturally occurring terrestrial and cosmic-ray-produced sources, and anthropogenic radionuclides to characterize the decay schemes of newly formed isotopes and analyze environmental and biological samples.