Teacher preparation programs (TPPs) have received a great deal of policy and research attention of late. And despite the commonsense notion that preparation for formal classroom responsibilities should improve the readiness of teacher candidates, the value of formalized preservice teacher education is unclear. In this review of the quantitative evidence about TPPs, I find that most studies show only minor differences in the value added of teachers who graduate from different programs, and that there are only a few studies that focus on the association between the features of teacher preparation and teacher workforce outcomes. The lack of evidence on the importance of the features of teacher preparation is primarily due to data deficiencies: data often do not permit connections between TPP features and teacher workforce outcomes. As a consequence, feedback loops that could theoretically provide TPPs with actionable information about program design typically do not exist.