This article describes the development of a Spanish-spelling measure designed to assess the progress made by Spanish-English bilingual children from Grade 2 to Grade 5. Different stages of measure development are described, such as the selection of the focus features, the pilot phase of the assessment, and the finalizing of the operational version. Two underlying attributes characterize the spelling measure described here. 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 Spanish-spelling ability over the full sequence of the upper elementary grades. Second, it is "contrastive," as it was designed to detect some areas of potential crosslinguistic influence from English to Spanish. 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 5 tables and 4 figures.)