The activity of progressing from a specification of some required program to a description of the program itself. Most phase models of the software life cycle recognize program design as one of the phases. The input to this phase is a specification of what the program is required to do. During the phase the design decisions are made as to how the program will meet these requirements, and the output of the phase is a description of the program in some form that provides a suitable basis for subsequent implementation.

Frequently the design phase is divided into two subphases, one of coarse architectural design and one of detailed design. The architectural design produces a description of the program at a gross level; it is normally given in terms of the major components of the program and their interrelationships, the main algorithms that these components employ, and the major data structures. The detailed design then refines the architectural design to the stage where actual implementation can begin. See also program design language.