The purpose of this case study was to understand engagement in action research as a form of professional development for practicing principals and the subsequent knowledge constructed as a result. The research team followed five principals through 1 year of professional development, which focused on supporting each principal in the design and implementation of an action research project on one's own leadership practice. Data sources included artifacts, field notes, video- and audiotaped transcriptions of meetings, and interview transcripts. Two assertions with supporting evidence are reported. First, the high-stakes testing environment and era of accountability dramatically shaped the topics of principals' inquiries and the ways that they understood the inquiry process. Second, collaboration with other principals, regardless of felt time constraints, played a critical role in the ways that principals made sense of the action research process as well as their own practice as administrators, and it precipitated deeper thinking about school and administrative practice. This study revealed that action research provides administrators with opportunities to engage in meaningful professional development that affects their own practices, the practices of other principals, and those within their school communities, thus making the act of reflection systematic and intentional. (Contains 1 table and 1 note.)