None of the most well-established indices within curriculum-based measurement of writing (CBM-W) assess students' use of vocabulary within writing. Yet vocabulary knowledge is fundamental to writing, as well as the other three domains of English language proficiency (reading, speaking, listening), and it is particularly crucial for English learners (ELs). This dissertation study explored reliability, validity, and classification accuracy for two types of CBM-W vocabulary indices for ELs: Number of Different Words (NDW; a production-dependent index) and Corrected Type Token Ratio (CTTR; a production-independent index). One-hundred and seventy-five second grade ELs were administered six monthly CBM-W probes and a state-issued English Language Proficiency test, which included scores for all four language domains. Results were similar for both indices, but preliminary findings indicated that NDW was more reliable and had slightly stronger classification accuracy than CTTR. For NDW, delayed alternate form reliability was moderate. NDW criterion validity correlations were variable, but mostly moderate with the writing and reading criterion scores, and mostly weak with the listening and speaking scores. Classification accuracy was weak for overall English language proficiency status, but moderate with the writing domain, and to a lesser extent, the reading domain. Results are contextualized within an MTSS framework for supporting the diverse language needs of ELs in second-grade. [The dissertation citations contained here are published with the permission of ProQuest LLC. Further reproduction is prohibited without permission. Copies of dissertations may be obtained by Telephone (800) 1-800-521-0600. Web page: http://www.proquest.com.bibliotheek.ehb.be/en-US/products/dissertations/individuals.shtml.]