An operational theory of design and manufacturing is developed which formalizes the design problem and addresses some of its major issues. We first formulate the problem in its most general form and then present a single design axiom. The axiom in effect asserts that the optimum design process minimizes the total expected cost of design and manufacturing. Since the cost is a direct function of the solution procedure, our focus is shifted from the product to the solution procedure. The design axiom is analogous to the principle of minimum energy in physics. In this case the energy equivalent is the total cost and the path of minimum energy is the trajectory of the proposed design vector defines the path. The design axiom then states that the optimum design will follow a certain optimum trajectory of minimum expected cost. One of the direct benefits in formulating the problem in operational terms is that a wealth results from systems theory may now be used to derive useful methodologies for the design/manufacturing process. In the present paper the consequences of the axiom are delineated and general principles of design are derived as a direct outcome of the design axiom. The general principles available to the designer can now be defended on a single principle, thus making possible the rational comparison of alternative schemes.