Of some 340 kinds of atoms that have been found in nature, about 70 are radioactive. Three families of radioactive isotopes[4]—the uranium, thorium, and actinium series—produce a large proportion of the natural radiation. Other radionuclides[5] occur singly, rather than in families, and some of them, such as potassium-40 and carbon-14, are major contributors of natural radioactivity. Traces of natural radioactivity can be found, in fact, in all substances on earth. When man began experimenting with atomic fusion and fission, he placed in his environment—across vast landscapes, in the oceans, and in the atmosphere—measurable additional amounts of radioactivity. These additions were composed of the longer-lived members of some 200 kinds of atomic radiation. Although the additions constituted but a fraction of the background burden, they represented the first alteration of the radiological balance that had existed since the early ages of the planet. Thus it became necessary to determine what the impact of such a change might be. In the process of inquiry, these ideas emerged: 1. The addition of man-made radioactivity presents the possibility of delayed or cumulative effects. Long-term studies, geared to the assessment of biological effects from extremely low radioactivity, are essential. 2. The addition of radioactivity makes possible broad-gauged studies to trace the movement and concentration of radionuclides in the environment. These studies, in turn, can disclose new information on biological complexes and mechanisms. [Illustration: _A flying atmospheric physics laboratory studying concentration of radionuclides over an Atomic Energy Commission laboratory. Instrument pod under wing samples air to provide a visual record of radioactivity._] [Illustration: _Transferring a sample of water taken from the depths of the Columbia River for radiochemical analysis in a laboratory._] The quantities of low-level long-lived radioactivity already released into our environment will provide materials for future studies covering decades. Further, because radioisotopes are chemically similar to nonradioactive forms, observation of their biological fate will provide clues to the transport, concentration, dilution, or elimination of many other kinds of man-made toxic agents and contaminants of the environment. Operating Concepts [Illustration: _Oceanographers bringing aboard a 50-gallon seawater sampler from the ocean depths find it a difficult task, even in moderate seas. This photo was taken aboard the R. V. Crawford in the Atlantic._] Environmental problems are best approached in the environment itself, where all the natural variables and unknowns are present. Laboratory work is essential, but no laboratory can carve from nature or reproduce artificially all the complexities of the natural environmental “laboratory”, the ecosystem.[6]