Concepts and findings from research on identity development are employed to better understand why current science teacher preparation programs are failing to prepare teachers who are able and choose to implement the vision for science education articulated in professional standards. Identity theory is used as a theoretical lens to make sense of and better address some of the unique challenges of becoming a reform-minded science teacher, a professional identity that does not reflect the common norm in the profession; these challenges include the emotional risk and possible need for "repair work," lack of familiarity with and buy-in into complex practices of inquiry, and the need for opportunities to participate in competent practice and have this participation acknowledged. Two basic design principles for science teacher preparation are identified as a result of this analysis: (a) the need to create safe places and scaffolded ways for beginning science teachers to try on and develop their identities as reform-minded science teachers, which may include capitalizing on the unique opportunities of practice teaching in out-of-school contexts; and (b) the need to offer opportunities to be recognized, by self and others, as reform-minded teachers through ongoing, structured, and supported reflection. (Contains 1 table.)