This article describes the development of an English spelling measure designed to assess the progress made by Spanish-English bilingual children from Grade 2 to Grade 5. Different stages of developing the measure are described, such as selecting the focus features, the prepilot and pilot phases, and the operational version. Two underlying attributes characterize the spelling measure described in this article. First, it is "developmental," meaning that it contains a wide variety of features and items that differ according to spelling difficulty, such that the assessment is able to measure the growth of English spelling ability over the full sequence of the upper elementary grades. Second, it is "contrastive"; that is, it was designed to detect some areas of potential cross-linguistic influence from Spanish to English. The combination of these two characteristics makes this spelling measure a unique tool for assessing the development of spelling ability by Spanish-English bilingual children. (Contains 7 tables, 4 figures, and 3 footnotes.)