Descriptive assessments are useful for identifying environmental events occurring before and after problem behavior but are not reliable or valid for determining the functional relationship. Functional analyses that systematically manipulate environmental conditions are needed to accurately identify the variables maintaining problem behavior so that effective treatments can be developed. A least restrictive assessment hierarchy is not necessarily best, as indirect assessments have poor reliability and descriptive assessments have high rates of false positives and negatives. Open-ended interviews followed by functional analysis is generally the best approach.