This paper suggests a conceptual framework and a language for the study of planning theory. It uses the approach developed by the Churchman-Ackoff school of thought in systems theory and builds a network of interrelated concepts leading to a general definition of planning, to a description of its main characteristics, and to an analysis of planning theories.The second and third sections establish a basis of structural and functional concepts to be used in the fourth section in a definition of planning and an examination of other concepts closely related to it. Section five interprets the work of a few planning theoreticians using the framework and language developed in the paper. The last section offers some suggestions for further research. © 1973.