The study described here examined the professional communities of teachers in American high schools. Data were derived from a series of observations and interviews undertaken over a 3-year period in six high schools in Michigan. Based on these data, the study developed a working definition of the term "professional community," described the professional communities observed in the schools studied, and formulated some general guidelines that can be used by policymakers and practitioners to nurture and promote the development of strong professional communities in schools. Recent research and policy analyses in education suggest that when teachers within a school form tightly knit professional communities there are positive benefits for students such as experiencing a higher quality of teaching, becoming more engaged in schooling, and having higher academic achievement. Within these communities teachers: (1) share a common core of educational values; (2) feel a strong sense of collegiality and have intensive collaborative relationships, both of which factors support continuing professional development and improved mastery of teaching; and (3) have a strong commitment to improving the work setting. (LL)