Student outcomes are of central importance in teaching effectiveness research (TER) because they serve as the criterion for inferences on teaching. This paper, using a systematic literature review, examines how carefully researchers conceptualize and operationalize student outcomes in the field of TER by analysing (a) the choice of outcomes, (b) the development of outcome measures, and (c) the existing evidence for the quality of outcome measures (reliability and validity) in TER studies. The results indicate a lack of explications of the theoretical grounding and a lack of validity evidence in many TER publications. This questions the interpretability of prior findings on the effects of teaching on learning. Approaches to addressing these issues in future studies are mapped out.