Teacher quality, along with the school context, classroom practice, and student mathematics achievement, are the key components in Clarke and Hollingsworth's (2002) model that depicts teachers' professional growth as an ongoing, dynamic, and interactive enterprise. Comfort and preparedness are dispositions that, along with teacher knowledge, are aspects of teacher quality. This model provides a basis for proposing a special causal mechanism: professional development designed to help teachers understand and respond to individual students' strengths and needs should support classroom equity by increasing teachers' preparedness and comfort, which in turn enhances the likelihood of teachers' engaging in differentiated instruction to support all students. This paper draws from two randomized controlled trials of the "Math for All" professional development program, which show consistent, positive results of the program on elementary school teachers' reports of comfort and preparedness to teach mathematics to diverse students--across modes of delivery, grade levels, and, to a lesser extent, locales. We also found comfort and preparedness to be modestly but consistently related to self-reported mathematical instructional practices across both studies. These results point to the consistency of the positive effects across sub-samples, and to the scalability of the "Math for All" program.