Teachers involved with professional-development opportunities inevitably differ in their content knowledge, access to resources, and instructional practices. The purpose of this study was to investigate how a standardized assessment observation tool, selected to gather summative information for grant-evaluation purposes about preschool teachers' early literacy instruction environment and practices, could be used to guide the implementation of an early literacy peer-coaching professional-development program involving 23 teachers in 5 preschools. Data were obtained from three sources: baseline and end-of-project data for each teacher on the standardized observation tool, interviews with codirectors and coaches, and documents provided by codirectors and coaches. Results indicate that a standardized observation tool can be used formatively in three ways: (1) to guide decisions about materials purchases; (2) to adjust professional-development workshop sequence and delivery; and (3) to guide coaches as they work one-on-one with teachers. (Contains 5 tables.)