Older adults, individuals 60+ years of age, make up an increasingly significant percentage of the population served by mental health clinicians. Thus, it is critical clinicians are equipped to conduct appropriately tailored and thorough diagnostic interviewing that considers the diverse needs, experiences, and contexts of older adults. This chapter provides an overview of key considerations for conducting a diagnostic interview with an older adult, including basic principles for assessment with older adults, prevalence of mental health conditions, contextual considerations (e.g., medical and cognitive conditions; life changes), and guidelines for formal assessment. Cultural considerations are outlined, including the influential nature of ageism. The concept of balancing awareness of diversity in aging with knowledge of core concepts to consider in working with older adults is demonstrated throughout. Key points are illustrated by a case example of an older adult referred for a diagnostic interview to address questions about perceived memory and mood changes. Ultimately, the chapter aims to guide mental health professionals to provide principle-guided, competent, and culturally responsive care to older adults engaging in a diagnostic interview.