During the twenty years the authors have been associated with the field of radiation ecology, there has been a diversified and increasing use of radionuclides in applied and basic biological research. Prior to the advent of the atomic age in the 1940s the use of radionuclides as tracers was initiated, and following that period one observed a dramatically increased use in many disciplines. Concurrent with this increase there appeared many books and articles on radionuclide tech niques useful to biologists in general. Although only a few ecological applications were evident in these early years, ecologists were quick to see the opportunities available in their field. In the United States, major centers for such activities included Oak Ridge National Laboratory and the U. S. Atomic Energy Com mission's Savannah River Plant. At Oak Ridge National Laboratory Dr. Stanley I. Auerbach, director of ecological activities, encouraged with remarkable suc cess the use of tracers by his associates. Dr. Eugene P. Odum had the foresight to see that radionuclide tracers provided the means to solve many problems of interest to ecologists. Consequently, his research included some unique radio tracer applications at the Savannah River Plant. In addition he encouraged others involved in ecological activities at the Savannah River Plant to do likewise. Ecologists such as Dr. Robert C. Pendleton at the U. S. Atomic Energy Com mission's Hanford Works applied radionuclides in their research. To these early investigators and to those who followed we owe the oppportunity to write this book.