This paper reviews the research leading from the GUIDON rule-based tutoring system, including the reconfiguration of MYCIN into NEOMYCIN and NEOMYCIN's generalization into the heuristic classification shell, HERACLES. The presentation is organized chronologically around pictures and dialogues that represent turning points and crystallize the basic ideas. Intended to collect important results in one place, the paper also makes some observations about research methodology, including abstracting or generalizing terminology to incorporate another specific domain, separating the domain model by identifying and justifying procedural sequences, and justifying domain relations in terms of underlying constraints and patterns. Twenty-two figures are provided. (41 references) (Author/EW)