Many teacher education researchers have expressed concerns about the lack of rigorous impact evaluations of teacher preparation practices. I summarize these various concerns as they relate to issues of internal validity, measurement, and external validity. I then assess the prevalence of these issues by reviewing 166 impact evaluations of teacher preparation practices published in peer-reviewed journals between 2002-2019. Although I find that very few studies address issues of internal validity, measurement, and external validity, I highlight some innovative approaches and present a checklist of considerations to assist future researchers in designing more rigorous impact evaluations.