Discussing career decision making (CDM), career guidance, a computerized system of career guidance, and the interplay among them, this book describes the way people sort themselves, or are sorted, into educational and occupational options. The options represent the content of this book, and the sorting represents the process. The sequence of decisions may extend over a lifetime, but several crucial choice-points tend to occur at predictable stages in a career. Career guidance is a professional intervention in CDM; "professional" implies that practitioners conform to a standard of ethics, knowledge, and competence beyond what may be offered by other intervenors. Guidance is partly an art, but it is also partly a science -- at least an application of science, based on a synthesis of logic and evidence derived from research. The computerized System of Interactive Guidance and Information (SIGI) is a designated guidance "treatment," clearly defined and specified. It was developed according to an explicit model, derived from a particular rationale for guidance, using modern technology to amplify the practice of career guidance. The current version -- called SIGI PLUS TM -- is being used at more than a thousand colleges and universities, as well as secondary schools, libraries, corporations, community-based organizations, and counseling agencies. These three interdependent topics are treated in a progression: from a theory of CDM to a rationale and a model for guidance to the design and development of a system. This book weaves together theory (principles, propositions, rationales, and models), research and development. The product of that development, SIGI, helps to define theory, to exemplify it, and to test it.