Educators are realizing that individuals working in isolation can't adequately address the teaching and learning problems that face us today. Collective action networks are needed. Sharing networks use collective energy to support individual action and agency, whereas execution networks typically address complex problems that require sustained collective action to resolve. One kind of execution network--networked improvement communities (NIC)--can be of great value to educators as they tackle complex problems of practice. The story of the formation of the Mathematics Teacher Education Partnership (MTEP) illustrates how catalyzing an NIC is different from forming a sharing community and why a formal infrastructure is required for sustained collective action.