This paper draws upon data from a longitudinal, multi-site, mixed methods project which found that commitment and resilience are fundamental to teachers' effectiveness, and that variations in professional, personal and workplace conditions in different professional life phases affect these. It found also that teachers do not necessarily learn through experience; that expertise is not acquired in an even, incremental way; and that teachers are at greater risk of being less effective in later phases of their professional lives. The paper develops these findings. Moreover, it argues that the contexts for teachers' professional learning and development are, by definition, different from those who do not work in human service organisations, since teachers are essentially engaged in work which has fundamental moral and ethical as well as instrumental purposes. Their capacity to exercise these effectively relates to their ability to manage positive and negative "scenarios" in different professional life phases. It suggests, therefore, that to be effective, professional learning opportunities must be designed which take account of the personal, workplace and external scenarios which challenge their commitment to these core purposes. (Contains 4 figures.)