Using a multisite cluster-randomized controlled trial design, we evaluated the effect of the first year of a three-year teacher professional development called Cognitively Guided Instruction (CGI) on third -fifth grade students' mathematics achievement. Teachers in the intervention condition participated in nine days of professional development focused on students' understanding of fractions and multiplicative reasoning. Five days occurred in the summer and four days occurring during the school year. The CGI 3-5 program was found to significantly impact student mathematics achievement (p =0.007). The point estimate of the main effect of the CGI program on student achievement (g = 0.18) was greater than many of the most effective teacher professional-development programs that have been subjected to rigorous evaluation (Kennedy, 2016).