Two primary purposes guided this quasi-experimental within-teacher study: (a) to examine changes from baseline through 2 years of professional development (Individualizing Student Instruction) in kindergarten teachers' differentiation of Tier 1 literacy instruction; and (b) to examine changes in reading and vocabulary of 3 cohorts of the teachers' students (n = 416). We observed teachers' instruction and assessed students on standardized measures of vocabulary and word reading. Results suggested that teachers significantly increased their differentiation and students showed significantly greater word reading outcomes relative to baseline. We observed no change for vocabulary. The results have implications for supporting teacher effectiveness through technology-supported professional development.