Spanish speakers in the United States are a steadily increasing population, up by 233% since 1980. Given the growing population of dual language learners (DLLs) and the large numbers of Spanish-speaking children enrolled in pre-kindergarten programs, addressing the educational needs of preschool-aged DLLs has become a national imperative. Specifically, the intersection of this growing population and the dearth of appropriate assessment tools to evaluate DLLs early language and literacy skills creates a need for assessments that accurately measure preschool performance. This manuscript reports on the iterative design process of a measure of Spanish phonological awareness for preschool-aged DLLs: Spanish Individual Growth and Development Indicators (S-IGDI) Primeros Sonidos. We employed measure design framework to develop the measure and tested item function within a study of 970, 4-5 year old DLLs. Results, including item level analyses and evidence regarding construct and criterion validity are reported. [This article was published in "Educational Assessment" (EJ1205857).]