The Technology Enhanced, Research Based Instruction, Assessment, and Professional Development (TRIAD) was developed by Clements and Sarama (2009) in order to support low-income children's mathematical development in the preschool years through professional development for preschool teachers in mathematics. TRIAD includes a classroom component (i.e., a mathematics curriculum entitled Building Blocks), a home component (mathematics activities and materials for families), and a professional development package consisting of workshops, mentoring, and online resources for its teachers. This study examined the effects of TRIAD and its effectiveness in increasing the quantity and quality of preschool teachers' mathematics instruction. First, the study examined whether preschool teachers who participated in TRIAD implemented mathematical activities more often and with higher quality in their classrooms compared with teachers who were not trained. Second, within the group of teachers who participated in the TRIAD professional development program, the study examined the effects of the amount of exposure to professional development on the amount and quality of mathematics activities teachers provided for their students. Third, individual contributions of various TRIAD components on preschool teachers' mathematics instruction were investigated. This article summarizes the findings and implications of the study.