Validity and reliability issues in standardized testing of students of limited English proficiency (LEP) were studied. Existing data from four different school sites were obtained for LEP and non-LEP students for three different standardized tests, the Stanford Achievement Tests (Ninth edition), the Iowa Tests of Basic Skills, and the Language Assessment Scale. Several different analyses were performed on the available data, including descriptive statistics by LEP status, analyses of internal consistency of the test items by LEP status, and analyses comparing the structural relationships of the instruments across LEP categories. Analysis results are consistent with the literature and indicate that: (1) student English language proficiency is associated with performance on content-based assessments; (2) there is a performance gap in content assessment between LEP students and their native English-speaking peers; and (3) the performance gap between LEP students and non-LEP students increases as the language load of the assessment tools increased. (SLD)