This study reports on the design and validation of a vertically equated assessment of academic vocabulary that generalizes to a meaningful corpus of words and is measured on a developmental scale: the Test of Academic Vocabulary in English (TAVE). The study builds on previous pilot work and uses a larger sample of students who are English learners (ELs) and non-EL students in grades 3 to 8 (n= 2,238) from a large urban Southwestern region, and describes the rationale and process of corpus and assessment development. A review of the findings from the study found the academic vocabulary construct to be unidimensional and to have both strong reliability and criterion validity. The TAVE was also able to discriminate performance by grade level in lower grades. For research, this study identifies a developmental metric where student scores not only generalize back to a meaningful corpus of words found in academic texts, but also offers specific expectations about which words students would know in the corpus. For practice, this study offers a tool that provides scores that are directly comparable across grades and could potentially be used to track growth across both the short and long term.