Determining how small an amount of a given pesticide or drug can be detected in plant or animal tissue has a profound effect on the commercialization of the compound being investigated. This effect lies within the complexities of governmental administration of pesticide and drug regulations, and is not a subject of this discussion. Suffice it to say, there is a premium in ascertaining the absence of a residue within the confines of a given analytical method, and a seeming compulsion on the part of investigators to employ ever more sensitive methods. The quest for the absence of pesticides and drugs has given rise to contradictions and misunderstandings which need not have occurred. A clarification of terms and calculations will be presented in the discussion to follow. The proposals it contains have withstood the test of a variety of pesticides, drugs, and biological tissues. A cautionary note should be added: for the sake of clarity, there is a scattering of very minor oversimplifications. They have been made to eliminate distractions from the clarity of the presentation. Because it is felt that these unstated complexities are not contradictory to the proposals in this paper, the experienced analytical chemist can make his own mental reservations.