States use standards-based English language proficiency (ELP) assessments to inform relatively high-stakes decisions for English learner (EL) students. Results from these assessments are one of the primary criteria used to determine EL students' level of ELP and readiness for reclassification. The results are also used to evaluate the effectiveness of and funding allocation to district or school programs that serve EL students. In an effort to provide empirical validity evidence for such important uses of ELP assessments, this study focused on examining the constructs of ELP assessments as a fundamental validity issue. Particularly, the study examined the types of language proficiency measured in three sample states' ELP assessments and the relationship between each type of language proficiency and content assessment performance. The results revealed notable variation in the presence of academic and social language in the three ELP assessments. A series of hierarchical linear modeling (HLM) analyses also revealed varied relationships among social language proficiency, academic language proficiency, and content assessment performance. The findings highlight the importance of examining the constructs of ELP assessments for making appropriate interpretations and decisions based on the assessment scores for EL students. Implications for policy and practice are discussed.