Diagnostic assessments are intended to support teachers' instructional decision making by providing instructionally relevant information. In this article, we propose that using cognitive theories of learning to design diagnostic assessments can provide teachers with two diagnostic outcomes: (a) the location of a student's thinking within the learning process, and (b) identification of misconceptions and errors that may underlie student performance. We focus on two test design considerations that may support these diagnostic inferences, specifically (a) sequencing item content to align with learning progressions and (b) constructing multiple-choice items with diagnostically relevant distractors. We provide an illustrative example from middle-school mathematics.