The processes of change in teachers' practice have been the subject of many studies in educational research. We here describe a case study of long duration (in a course of time of 9 years) that has allowed us to gain some insight into how they occur and the underlying obstacles making the processes so slow. We take the professional development of a science teacher as involving the integration of reflection and classroom practice. In this theoretical framework, the so-called complexity hypothesis is of particular significance. The teacher was immersed in diverse learning environments, including an action research program. The results showed her reflections and classroom practice to be in constant evolution, as also were the obstacles to her professional development and an obdurate core of practical theories that she brought with her about the exclusive use of closed problems strictly within the discipline being taught. By the end of the study, her use of activities focused on socioscientific issues was in its beginnings. Continued learning in practice with the community of her fellow teachers with the emotional support they provided and a solid knowledge of the content were revealed as essential for her professional development. (Contains 8 tables and 1 figure.)