INTERNIST-I was an expert system designed in the early 1970's to diagnose multiple diseases in internal medicine by modelling the behaviour of clinicians. Its form and operation are described, and evaluations of the system are surveyed. The major result of the project was its knowledge base which has been used in successor systems for medical education and clinical use. We also survey the effects of the project through these systems, and conclude that the most successful of them in the near future is likely to be Quick Medical Reference (QMR) when used as an "electronic textbook" of medicine.