Professional development is seen as one of the major levers for aligning science instruction in the USA with the vision put forth by national standards documents. Although there is a growing consensus regarding what constitutes effective professional development, there is little empirical evidence to support this consensus. This study examines the impact of professional development that is content-based, situated in classroom practice, and sustained over time on teacher attitudes, perceptions of preparedness, and classroom practices. It utilizes longitudinal data from the National Science Foundation's Local Systemic Change through Teacher Enhancement Initiative (LSC), collected from 42 projects over a span of 7 years. The professional development model used in the LSCs differed from previous initiatives in that it targeted all teachers in a jurisdiction and emphasized preparing teachers to implement project-designated instructional materials. Analyses of the data provide evidence that this model for professional development has an impact on teachers and their classroom practices. In addition, the analyses found that teachers' perception of principal support for "Standards"-based science instruction is an important predictor of these outcomes.