Diagnosis is an integral part of instructional decision-making. As the bridge between identification of students who may be at-risk for failure and delivery of carefully designed supplemental interventions, diagnosis provides valuable information about students' persistent misconceptions in the targeted domain. In this paper, we discuss current approaches to diagnosis in mathematics and highlight the strengths and limitations of each approach for making instructional decisions. We point to cognitive diagnostic assessments as an emerging solution for providing detailed and precise information about students' thinking that is needed to provide appropriate educational opportunities for students struggling in mathematics. (Contains 3 figures and 1 table.)