In this work we present the basic principles of metabolic control which we hope will serve as a foundation for the vast array of factual matter which the biochemist and the physician engaged in metabolic research must accumulate. Accordingly, we attempt to set forth these principles, along with sufficient explanation, so that the reader may apply them to the ever-expanding literature of biochemistry. If we are successful, this will provide a theoretical approach which can be applied to any given set of metabolic reactions. It is impossible to enumerate each and every biochemical reaction and pathway since such a work would be too unwieldy for efficient use. Rather, we hope our presentation of the principles of metabolic control will be sufficiently basic to be of lasting usefulness no matter how detailed biochemistry may become. We would like to be able to con- dense biochemistry into a theoretical biology that will not only allow for the general treatment of any given reaction but will enable predic- tions to be made as to the existence of necessary pathways and the con- sequences of altered control. Such is not possible today, but this may be accomplished in the future. We believe it is now possible to institute the beginnings of such a theoretical biology.