This paper examines how teachers develop and modify professional knowledge in response to experience and to new research knowledge. A discussion is presented on the interaction of propositional knowledge (the basic medium of classroom and research knowledge) and practical knowledge (the observable competence apparent in successful professional actions). Alternative definitions of professional expertise are examined prior to taking a close look at the issue of learning from experience, using data provided by several of the teachers who participated in the study. An analysis of how teachers learn from experience takes into consideration four broad principals of expertise: discrete technical skills, application of theory or general principles, critical analysis, and deliberate action. Summaries are presented of interviews with five teachers, of whom one was a preservice teacher, two were beginning teachers, and two were experienced teachers with Master of Education degrees. Data selected from the interviews suggest that there are tensions associated with the differences between research knowledge and practical knowledge and that this is a significant problem for beginning teachers. (JD)