In this article, I specify a conceptual framework for test validity research on content assessments taken by English language learners (ELLs) in U.S. schools in grades K-12. This framework is modeled after one previously delineated by Willingham et al. (1988), which was developed to guide research on students with disabilities. In this framework for research on ELLs, there are eight indicators of test comparability. Five of these indicators are measures of score comparability, while three indicators are measures of task comparability. To date, research has been conducted on six of the indicators of test comparability for content assessments taken by ELLs. For these indicators, findings from representative studies are summarized. For the remaining two indicators of test comparability for which no published research currently exists, I describe the types of research studies that are necessary to gather evidence to evaluate the comparability of content assessments for ELLs. (Contains 2 tables.)