Purpose : We conducted a retroactive analysis of teacher and student data from two randomized group trials and one well-controlled quasi-experimental group trial focused on improving pedagogical content knowledge (PCK) and student performance. Method : Seven teachers and 32 classes were investigated. PCK was measured using four variables: task selection, representation, adaption, and an aggregate variable called total PCK. Student data are reported as percentages of correct performance. Data are reported descriptively using effect sizes (ES). Results : The studies generated 35 ES across four teachers and one student performance variable. All ES exceeded the U.S. Department of Education’s What Works Clearinghouse .25 standard deviation criterion for a “substantively important” effect and all ES exceeded Cohen’s criteria of .8 for a large effect. Discussion : Findings from this study support a focus on professional development of teachers’ content knowledge as an evidenced-based practice for improving the PCK of teachers and in turn student performance.