Building on the papers in this special issue, this article uses modern conceptions of validity theory to provide a framework for considering the evaluation of teaching quality. The 3 facets of teaching quality focused on are domain conceptualization, evidence and inferences, and their evaluation. Domain definitions vary in their specificity with tradeoffs in their range of applicability and specificity of inference. Evidence collection can range from highly standardized assessments to observations that must attend to evidence from a myriad of classroom interactions. For all assessments, however, even the most standardized, different interpretations of assessment tasks can threaten the validity of score interpretations. The papers consider a range of processes that are designed to generate, support, and interrogate the validity of inferences based on assessment scores. A fundamental question underlying this type of measurement is whether differences in the quality of teaching that students experience can be causally attributed to the teacher.