This study addresses current dilemmas surrounding features of effective teacher professional development (PD). Using a theoretical framework that conceptualizes the complexity of teacher learning, this study investigates the interconnections among teachers' professional backgrounds, purposes for developing professionally, teaching contexts, engagement and participation in PD, and PD outcomes. Drawing upon the experiences of 16 case study teachers participating in a multi-year, multi-site environmental science literacy PD, this research uses a variety of data sources to better understand how teachers applied the concepts and practices from their PD program to the classroom context. Our analysis identified three forms of classroom implementation: "Integrated" incorporated practices across the curriculum, "Focused" attended closely to one interest area, and "Consistent" involved authentic instruction of the curriculum as designed. Findings suggest that teachers who demonstrated "Integrated" implementation were more likely to come to PD with a general desire for professional growth and engage collaboratively with colleagues. Implications include a deeper understanding of teacher learning to inform the design of effective PD.