Mobile device (e.g., smartphone) proficiency is becoming increasingly necessary to perform important everyday tasks, and inadequate proficiency can prevent groups of individuals such as older adults from obtaining the benefits of mobile computing and smartphone utilization. To facilitate mobile device training and research on barriers to mobile device use and adoption by older adults, Roque and Boot [1] developed the Mobile Device Proficiency Questionnaire (MDPQ). The current study is the first to assess the validity of the MDPQ based on confirmatory factor analysis and objective measures of their smartphone skills. In addition, it represents the first attempt to validate the MDPQ on a sample of older smartphone users. Results suggest that modifications may be necessary for the MDPQ to serve as a valid and reliable measure of proficiency among older adult smartphone users. In this sense, four important implications and recommendations for application of the MDPQ are discussed that could promote the general goal of ensuring that individuals of all ages can benefit from the use of smartphone devices and services such as mobile apps. INDEX TERMS Social implications of technology, human-computer interaction, cellular phones, construct validity, criterion validity, mobile device proficiency questionnaire, older adults, scale development.