English Language Learners (ELLs)--students whose second language is English and who are not fully proficient in English--constitute the fastest growing portion of the K-12 student population. Because many of these students tend to do poorly in school, teachers are encouraged to regularly use research-based practices to improve these students' academic achievement. Yet knowing which practices actually are research-based--that is, they are supported by research demonstrating impact on student outcomes--is not clear to many educators. In this series of four articles, the authors demystify what is and is not "research-based" as determined by two recent reports--"Developing Literacy in Second-Language Learners: Report of the National Literacy Panel on Language Minority Children and Youth" (August and Shanahan 2006); and "Educating English Language Learners" (Genesee et al. 2006)--and select publications that have appeared since. This four-part series of articles summarizes what research says about effective practices for ELLs--and what it does not say. The authors focus on several recent reviews of the research (August and Shanahan 2006; Genesee et al. 2006; Goldenberg 2008; Saunders and Goldenberg, in press), providing the key findings and explaining how those conclusions might inform classroom practice. This first article in the series covers some of the research on oral language proficiency. The authors summarize key conclusions about ELL's oral language proficiency from research and research reviews, and then provide a classroom scenario that illustrates effective strategies in practice. The authors close this article with an example of how a lesson might look in classroom practice. (Contains 1 table.)