The growing population of English language learners (ELLs) in U.S. schools and the low academic achievement of many of these learners have been the subject of much debate. A significant related issue is determining the sources of ELLs' difficulty, namely, understanding the distinction between learning disabilities (LD) and learning difficulties due primarily to contextual factors and second-language learning. This article addresses the future directions for research in this area, with an emphasis on the need to build consensus through converging lines of evidence. Issues of sample definition, study design, and methods are discussed; also emphasized in the article is that in order to answer questions about the sources of difficulty for ELLs who are struggling, an understanding of the trajectories and experiences of those not experiencing difficulties is needed. Throughout the article, parallels are drawn between this emerging field of research and the LD research conducted with monolingual English speakers. An example of a study for which the design, analyses, and dissemination relates to many of the issues raised in the article is presented.