Research suggests that learning is situated, social, and constructed. Teachers, as well as their students, learn through collaborative participation in meaningful activity. This study investigates the impact of a Writing Project Institute on teachers' learning and instruction, considering characteristics of professional learning that influenced the institute's impact. Interview and observational data suggest that the Institute impacted teachers' practice, their understanding, and their efficacy. Additionally, there was evidence that students' writing improved. Characteristics of the Institute that appeared to support this impact include: agency and autonomy, collaboration and social construction of knowledge, and active and purposeful learning. Examples of each of these characteristics demonstrate how these attributes might be incorporated into opportunities for professional learning.