Modern analytical methodology, especially radiotracer techniques, makes it possible to detect residues of a foreign compound in animal products at levels seemingly impossible just two decades ago. Unfortunately, the ability to detect "total radioactivity" in a substrate does not assure that the chemical nature of the residues will be elucidated or that their toxicological significance can be properly assessed. Detection without identification has created a number of problems for the residue chemist and toxicologist. Among these is the problem of classifying radioactive residues in a manner that is meaningful to other scientists and which inherently denotes certain characteristics of the residues involved. While such a system is as yet incomplete, a general classification scheme has evolved in recent years that provides a common ground for categorizing radioactive residues, be they known or unknown. The system consists basically of four categories of radioactive residues: (a) free metabolites (b) conjugate metabolites (c) bound pesticide residues and (d) natural constituents. For the most part, the terms are self-explanatory, but precise definitions remain a point of debate among scientists. This paper discusses th criteria for classifying radioactive residues as perceived by the author based upon works of residue and metabolism chemists, especially those dealing with pesticide residues. Utimately, classification is dependent on the identification of total radioactive residues, a situation which is virtually impossible with many xenobiotics. Sound scientific judgment must remain the key ingredient in determining just how far one must go in identifying radioactive residues of drugs, pesticides and other chemicals which may become a component of the human diet.