We address valid testing for English language learners (ELLs)--students in the United States who are schooled in English while they are still acquiring English as a second language. Also, we address the need for procedures for systematically developing ELL testing accommodations--changes in tests intended to support ELLs to gain access to the content of items. We report our experience developing and investigating illustration-based accommodations for ELLs in terms of five main activities: specification, design, production, evaluation, and revision. Our findings indicate that, to benefit from a given accommodation, ELLs need to possess certain skills that enable them to use it but that are not related to the knowledge assessed. Critical to developing effective accommodations is examining how their properties interact with the characteristics of the students and the characteristics of test items.