This double issue explores the Professional Development Schools (PDS) movement and its impact on schools, universities, and the people who lead, learn, and work in them. Part I of this theme issue answers many "what" questions about PDS including describing its historical roots, reviewing the literature on PDSs, clarifying the many models that exist across the country, and detailing how teacher educators' roles change during PDS work. Part II of this issue addresses the "so what" questions including generalizing about the conditions under which broad-based school-university partnerships can support PDSs that have positive results for teacher preparation and development, identifying barriers to reform and proposing needed policy changes and research initiatives, and examining the findings from work the National Foundation for the Improvement of Education has conducted with schools and teachers' associations across the country to suggest ways PDFs could improve results for teachers in schools.