We report on a three-year study of teachers' experiences of a major reform of the science National Curriculum for 14- to 16-year-olds in England. Teachers' responses to this curriculum reform were guided by: "personal" aims and biography; "internal" features of their workplace such as departmental collegiality; and "external" features such as educational policies outside of science. We argue that reforms should provide teachers with sufficient flexibility to allow them to adapt reforms appropriately to local contexts. Policy makers should also consider how different educational reforms might interact over time. Professional development activities should not be seen as opportunities to promote particular curriculum reforms; rather they should support teachers in developing an informed and critically reflective perspective on curriculum policy directives.