Purpose: The purpose of this research forum article is to provide an overview of typical and atypical development of English as a second language (L2) and to present strategies for clinical assessment with English language learners (ELLs). Method: A review of studies examining the lexical, morphological, narrative, and verbal memory abilities of ELLs is organized around 3 topics: timeframe and characteristics of typical English L2 development, comparison of the English L2 development of children with and without specific language impairment (SLI), and strategies for more effective assessment with ELLs. Results: ELLs take longer than 3 years to converge on monolingual norms and approach monolingual norms asynchronously across linguistic subdomains. Individual variation is predicted by age, first language, language learning aptitude, length of exposure to English in school, maternal education, and richness of the English environment outside school. ELLs with SLI acquire English more slowly than ELLs with typical development; their morphological and nonword repetition abilities differentiate them the most. Use of strategies such as parent questionnaires on first language development and ELL norm referencing can result in accurate discrimination of ELLs with SLI. Conclusions: Variability in the language abilities of ELLs presents challenges for clinical practice. Increased knowledge of English language learning development with and without SLI together with evidence-based alternative assessment strategies can assist in overcoming these challenges.