This longitudinal study explored change in teachers' knowledge of subjects they teach from preservice training through 17 years of professional experience. It followed secondary school science teachers in Australia, through sequences of individual interviews in which change in content knowledge (mainly energy-related) was probed primarily via concept profiles--a word-association method. Change was found to be multifaceted, with details of unused content fading from memory, alongside growth that results from improved understanding and reorganization of structure more than from accretion of new material. Across personal and professional life tracks that produce variation between individuals, development is facilitated by "critical mass" of teachers' knowledge and interest in their chosen disciplines of study and certification, whereas deficiencies tend to persist in the other subjects they are asked to teach. The required curriculum is the single most powerful determinant of teacher knowledge, serving as both its organizer and source. Based on the study, a three-phase model of teacher content-knowledge development is proposed, and the discussion highlights the need for career-long support for growth, even in teachers' major subjects where expertise is taken for granted. (Contains 4 figures, 5 tables and 3 footnotes.)