This study compared the efficacy of 3 approaches to professional development in middle school Earth science organized around the principles of Understanding by Design (Wiggins & McTighe, 1998) in a sample of 53 teachers from a large urban district. Teachers were randomly assigned to a control group or to 1 of 3 conditions that varied with respect to the conceptions of ideal curriculum use embedded within. Teachers either designed units of instruction, adopted units developed by expert Earth scientists and Earth science educators, or learned principled ways to adapt expert-designed curricula. Relying on data from surveys and independent ratings of naturally occurring teacher assignments, we used hierarchical linear modeling techniques to analyze the impacts of the professional development on how teachers planned and coordinated instruction. Our results suggest that to realize positive effects on both their planning and coordination of instruction, teachers need access to high-quality curriculum materials and professional development that helps them plan for principled adaptation of those materials. (Contains 10 tables, 3 figures, and 3 footnotes.)