Students with serious emotional disturbance (SED) are identified at relatively low rates, perhaps because of confusion about how to meet the federal definition's elements. This article addresses a practical aspect of the SED definition: how to determine if emotional disturbance has adversely affected educational performance. It is suggested that effects of emotional disturbance should not be equated with those of learning disabilities, especially if an emotional disorder begins late in a student's educational career or has a variable course. Three ways to determine whether an emotional problem has adversely affected educational performance are suggested: severe ability/achievement discrepancy, failure to master the curriculum, and chronic absence from school. Implications for professional practice are also addressed.