One-on-One Tutoring by Humans and Computers articulates the CIRCSIM-Tutor project, an attempt to develop a computer tutor that generates a natural language dialogue with a student. Editors Martha Evens and Joel Michael present the educational context within which the project was launched, as well as research into tutoring, the process of implementation of CIRCSIM-Tutor, and the results of using CIRCSIM-Tutor in the classroom. The domain of this project is cardiovascular physiology, specifically targeting first-year medical students, though the idea is applicable to the development of intelligent tutoring systems across populations, disciplines, and domains. This 5 year-long project was motivated by the belief that students need assistance in building appropriate mental models of complex physiological phenomena, as well as practice in expressing these ideas in their own words to fully develop those models, and experience in problem-solving to use those models effectively