English Language Learners (ELLs) are a dynamic and rapidly growing population within the California school system. In this age of greater accountability, teachers need access to the tools necessary for effectively reaching this growing subpopulation of students. In this study, A Developmental English Proficiency Test (ADEPT), a language assessment tool, is examined with regards to its effectiveness in assisting teacher candidates in their instructional planning for ELLs. Results from qualitative and quantitative data show that the ADEPT was effective in helping teacher candidates to plan and differentiate instruction for ELLs. Additionally, the ADEPT provided concrete data to report to school administrators and parents on students' progress. (Contains 1 figure.)