Ongoing site-based professional development is a much more complex endeavor than old-style teacher workshops conducted in isolation from the immediate context of the school. Sitebasedprofessional development asks teachers to identify and improve the components of their instructional programs that are not working, a process that is sometimes painful andarduous. External facilitators involved in site-based professional learning face the challenges of both the complexity of the work and unpredictability of school environments. Staff at the Appalachia Eisenhower Regional Consortium for Mathematics and Science Education have learned to increase teachers’ capacity to effectively teach math and science and improve student achievement through ongoing site-based professional development — and learned a number of lessons along the way that can benefit external staff developers working within schools. 1. INVOLVE