Principals can increase the instructional capacity of their schools by creating opportunities for teachers to work collaboratively as they use key resources to improve teaching and learning. The author looks at how the principals in two schools she studied organized teachers to work together. One school created an organizational structure that enabled teacher collaboration but fell short of changing teachers' ideas about how to use instructional resources, such as how to work together, or of supporting teachers in making instructional changes. The second school was far more successful at building instructional capacity. This school tapped into administrators as an important instructional resource and made teachers' instruction a source for teachers' ongoing learning. To build instructional capacity in their schools, principals need to create the right structures, conditions, and expectations. They also need to assemble the right teams and specify the focus of teachers' joint work. Collaborative work becomes an invaluable approach to improving school performance--provided that schools have designed these teams for learning.