This is a case study of a science teacher (‘Debra’) learning to teach, with special attention to her learning from experience. The case is unusual because Debra worked for eleven years as a research technician in chemistry before she shifted to teaching chemistry in a secondary school. Much attention is being given to ‘pedagogical content knowledge’ and to ‘reflection’ in discussions about how individuals learn to teach, and we believe this case makes an important contribution to how those terms are understood and used in working with student and first-year teachers. We find this case particularly interesting for its suggestions about possible ways in which professional knowledge is acquired. One unique feature of this case is Debra’s knowledge of chemistry not just from a student’s perspective but from extensive laboratory experience. A second unique feature is the role of the observer who was asked to offer assistance as he also gathered observational and interview data for understanding Debra’s teaching.