Why don't most well-meaning efforts to improve teaching and grow key teachers' capacity to act as instructional leaders show success? Kenneth Baum and David Krulwich think they know why: While we set up "collaborative" procedures and meetings in which teachers work together to explore effective instruction--critical friends groups, data discussions, PLCs, and so on--such meetings aren't truly collaborative. Teachers from different grade levels or disciplines are often paired. They don't share the same "defining work" (the creation and delivery of an actual lesson), so collaboration is superficial. The authors point out that in most fields, improvement happens as professionals work daily on their defining work (such as an actual case for lawyers) under the guidance of an "artisan"--a senior, skilled colleague. Baum and Krulwich describe the "Artisan Teaching Model" practiced at the Urban Assembly School for Applied Math and Science in which PD happens for all teachers through such defining work, enabling many teachers to learn to guide other teachers in the process. Teams of 2-5 teachers who teach the exact same subjects meet daily (with a selected "artisan" Team Leader) to create common learning objectives, weekly lessons, and assessments. They practice delivering lessons, observe one another, and revise lessons. The school avoids additional "PD" activities to free up teachers' time.