For many years, changing beliefs has been considered a prerequisite for changing classroom practices. However, professional development research has also shown that the opposite relationship is also true--change in practice can precede change in beliefs. This study investigated the effect of a one-year professional development program on in-service science teachers' instructional practices, beliefs, and their students' achievement. The professional development program specifically emphasized supporting middle school in-service science teachers to implement inquiry-based instruction. A non-random, single group, interrupted time-series, quasi-experimental design was used to test Guskey's model of staff development and the relationships among changes in teachers' instructional practices, beliefs and their students' achievement. Similar to Guskey's model, findings from this study show the importance that evidence of improved student achievement has on teachers changing their practices. Teacher change models emerging from our data did not find any links between general teacher change variables. However, factors such as teachers' focus on discourse and curriculum were found to be important components of inquiry-based instruction and teachers' contextual beliefs.