Comfort and preparedness are dispositions that along with teacher knowledge, comprise teacher quality (cf. Clarke & Hollingsworth, 2002). Professional development programs that help teachers understand and respond to individual students' strengths and needs can support equity in the classroom by building teachers' preparedness and comfort, which enhances the likelihood of teachers' engaging in differentiated instruction to support all students. This paper draws from two randomized controlled trials (RCTs) of "Math for All," which show consistent, positive results of the professional development (PD) program on elementary school teachers' reports of comfort and preparedness to teach mathematics to diverse students. We found comfort and preparedness to be modestly but consistently related to self-reported mathematical instructional practices across both studies. The fact that the results were similar across the two RCTs even though the modes of delivery differed (PD led by developers in RCT #1; by local staff developers and teacher leaders in RCT #2) also speaks to the scalability of the "Math for All" program.