It is widely recognized today that a teacher's surrounding professional community inevitably affects the kind of teaching that students are offered. However, until recently, it has been difficult to quantify and systematically study the nature of professional relationships in schools and districts. The articles in this special issue provide a much-needed collection of techniques to study the complex social interactions, or lack thereof, that shape teachers' professional communities. We relate the interactions described in the articles to our own experience working with and in nested learning organizations. Our intent is to illustrate how policies and procedures at any level can either enable or constrain actions at adjacent levels. (Contains 3 notes and 1 figure.)