One of the areas subsumed under the data-based decision making and accountability practice identified in the National Association of School Psychologists' (NASP) "Model for Integrated School Psychological Services" is to collect information on psychological and educational variables to make decisions at a number of levels of service delivery. The importance of engaging in ethically responsible assessment practices cannot be overstated given the high stakes associated with many of the interpretations that are drawn from assessment results by school psychologists. Differential item functioning (DIF) is one of the components that tends to appear in most technical manuals. The purpose of this paper is to explain DIF in a way that is understandable to practitioners who should be aware of DIF when working with various assessments and clients from different backgrounds. Measures of DIF are used to assess the fairness of tests to ensure the results produce an accurate picture of all examinees' abilities. Over the past 5 years, DIF has been a common measure used in many kinds of tests, but it is particularly important to use in test development for construct validation. The guidelines for DIF are presented to aid practitioners to evaluate DIF findings.