The number of English language learners (ELL) in schools continues to rise. However, statistics reveal that the majority of classroom teachers have no training in working with ELLs (NCES, 2011). Because of this, it is critical to understand how teachers can be successfully prepared to teach ELLs. Through in-depth inquiry, this study explored what kinds of knowledge four exemplary teachers of ELLs had, how they gained that knowledge, and how the knowledge was influenced by the context in which they taught--bilingual or monolingual. A knowledge base for what it takes to be a successful teacher of ELLs is presented. [Note: The citation provided on the full text (v2 Fall 2013) is incorrect. This article was published in v23 n2 Fall 2013.]