Beginning in the 1960s the interactive capabilities of computers were recognized as providing unprecedented opportunities for the development of computerized career guidance systems. Such systems offered individualized assistance in career decision making to large numbers of people at low cost. Recognizing that each user may present a unique combination of needs, experiences, circumstances, values, interests, skills, styles, and perceptions, a developer can enable a well-planned system to respond appropriately to individual differences. A system can store, retrieve, and manipulate vast amounts of information, putting great resources at the fingertips of each user, and it can bring many personal, occupational, and educational variables together, combining them in distinctive ways for each client. These capabilities do not ensure that the substance of any system will be of high quality. Much of a system's worth depends on its conceptual framework, the strength of its theory and rationale, and the coherence of its functions and structures. The "black box" of theory underlying every system should be opened up and exposed to the light of scrutiny and evaluation. Examples from the principles, research, model, and structures for the System of Interactive Guidance and Information (SIGI) and SIGI PLUS are used to illustrate the links between theory and practice. (Contains 21 references.) (Author/SLD)