As the number of dual language learners (DLLs) increases globally, it is critical that well-developed questionnaires are available to researchers and educators that capture the characteristics and language experiences of preschool DLLs and their families. To fill this need, the Center for Early Care and Education Research-Dual Language Learners (CECER-DLL) Child and Family Questionnaire was developed. This article describes the four-step iterative process that was used to create the questionnaire, with a particular focus on the cognitive interviews that were conducted to establish the validity of the items. Findings from the cognitive interviewing process are presented which demonstrate the importance of this step when creating questionnaires that are valid for diverse populations. The development process of the CECER-DLL Child and Family Questionnaire, which is now available to the field, serves as an example of how to create well-designed questionnaires for other populations of DLLs in the United States and beyond. (author abstract)