Convening professionals--physically or virtually--is necessary but certainly not sufficient for meaningful collaboration and networking to occur. Professional development (PD) must be designed and carried out in ways that encourage a culture of sustained collegiality (Loucks-Horsley et al. 2003). This article presents seven principles for creating and implementing collaborative PD. It summarizes lessons learned from a long-term evaluation of federally-funded science and mathematics teacher PD projects across the state of Missouri. (Contains 3 online resources.)