Interest in improving the quality of professional development in this age of educational reform has intensified as a growing body of research suggests that teaching practices matter in terms of student achievement. Some have argued for embedding professional development in the context of teachers' work in order to transform both teaching practices and the structures and cultures of schools in which teachers practice. These changes are necessary so that teachers can develop innovative teaching practices. Grossman, Wineburg, and Woolworth (2001) developed a set of markers to guide the formation of a workplace-based professional community. The markers, which identify issues that should be addressed when attempting to change teaching practice within the context of schools, may be helpful in developing other professional-development activities in the workplace. One such professional-development activity that may be useful in an environment of trust is what one refers to as "open lessons." In this article, the authors describe the theoretical underpinnings of the workplace-based professional community that Grossman, Wineburg, and Woolworth developed. The authors also discuss how open lessons could help build a professional community.