The purpose of this manuscript was to help researchers better understand the causes and implications of differential item functioning (DIF), as well as the importance of testing for DIF in the process of test development and validation. The underlying theoretical reason for the presence of DIF is explicated, followed by a discussion of how to test for the presence of DIF using logistic regression and DIFPACK, which includes SIBTEST, PSIBTEST and Crossing SIBTEST. This manuscript stresses the importance of conducting DIF analyses using a "priori" hypotheses whenever possible. However, the example that is provided, to show researchers and practitioners how to conduct a DIF analysis, utilizes an exploratory DIF analyses paradigm which may often be needed in practical DIF applications. This example uses PSIBTEST to test for DIF, using data from an international assessment that includes a mixture of polytomous and dichotomous items. In addition to demonstrating how to test for DIF, this manuscript demonstrates how researchers and practitioners should interpret the results of such an analysis. Finally, implications for both practitioners and researchers are provided. (Contains 2 notes, 2 tables, and 3 figures.)