Learning progressions are the successively more sophisticated ways of thinking about an idea that follow one another over a broad span of time. This review examines the research on science teachers' pedagogical content knowledge (PCK) in order to refine ideas about science teacher learning progressions and how to support them. Research published between 1986 and 2010 relevant to science teacher learning and PCK was examined for what ways teachers' knowledge becomes more developed and what appears to be the sequence. Analysis indicates that it is helpful for teachers to think about learners first, then to focus on teaching, and points out the essential role of reflection for teachers to rearrange their ideas in ways that develop their PCK. This review takes a unique approach to thinking about research on what science teachers learn and can support teacher educators in designing professional programs that support beginning and advanced learning for science teachers. (Contains 3 tables.)