Evidence-based assessment (EBA) and evidence-based practice are gaining attention and application in clinical psychology (Hunsley and Mash, Annual Review of Clinical Psychology 3:29-51, 2007; Youngstrom and VanMeter, Clinical Psychology: Science and Practice 23:327-347, 2016) and recently in school psychology. This commentary explores EBA in school psychology with implications for research, training, and clinical practice. Fundamental measurement principles and psychometric knowledge and application are essential for school psychologists to ethically practice and are core elements for EBA. It is hoped that there will be a dramatic increase in diagnostic and treatment utility studies reported in the peer-reviewed literature to help guide EBA in school psychology practice and that school psychology will be substantively guided by EBA despite challenges of inconsistently or inadequately defined disabilities.