Since 2009, our center for teaching and learning has offered an intensive Course Design Institute (CDI) several times each year, which has now been completed by more than 600 teaching faculty, staff, and Graduate Teaching Associates from The Ohio State University. To better understand the impact of participating in a CDI on participants' teaching, this study utilizes qualitative data drawn from five years of participant feedback gathered on the last day of each CDI, as well as from focus groups conducted with CDI graduates in the years following their participation. The results show that participating in the CDI helps instructors become more expert teachers, primarily through improving their metacognition about teaching. Closely examining the nature of the CDI suggests that the structure and content together may be particularly instrumental in helping participants develop their metacognitive abilities. Taken together, this research suggests that course design may be a highly efficient method for the long-term development of expert teaching.