Attempts to implement the professional development school (PDS) are frustrated by a lack of consensus about the meaning of the defining characteristics of these new schools. An explication of the common criteria is given that shows how each is an implication of the others and how the PDS is not a laboratory or demonstration school. Particular attention is given to the nature of inquiry in the PDS and its relationship to other forms of academic inquiry. Finally, the lessons from the 4-year effort to implement the PDS features in selected schools in Delaware is briefly described.