This paper presents initial findings from a 2-year study to identify the core knowledge and skills necessary to teach in a standards-based system. The study was part of the Standards-Based Teacher Education Project (STEP), which focused on the development of frameworks and materials for preservice and practicing teachers. High school teachers and higher education faculty formed "co-development teams" in each of six disciplines committed to implementation of standards-based instruction and assessment in one or more of their classes. In collaborative work groups, they shared pedagogical practices, broadened content knowledge, and learned new ways to engage students in learning. Culminating statewide institutes in the spring of 1997 and 1998 provided the opportunity for co-development teams to cross-score classroom performance assessments, determine levels of student proficiency, and verify one another's judgments of proficiency. This paper summarizes initial findings from participants' reflective inquiry. The study found that the voices of STEP teachers engaged in the actual implementation of standards provided clear imperatives for those who formulate educational policy and design teacher preparation and professional development programs. The overarching imperative is that teachers are the primary agents of school reform and should be included in all stages of design, implementation, and improvement of the standards-based system. (Contains 47 references.) (SM)