Substantial shifts have occurred in research and clinical practice in the last two decades towards improving identification, diagnosis, and understanding of autism in adults. One important aim of assessment of autistic people in adulthood is to identify, assess, and diagnose autism in adults who were not identified in childhood. Another important aim is to assess, understand, and support autistic adults’ and older adults’ strengths, needs, preferences, and to explore risk and enabling factors affecting their daily functioning, employment, education, social participation, health, and overall wellbeing and quality of life. Considering these two key aims of assessment in adulthood, in this chapter we critically summarize current guidance for the assessment and diagnosis of autism in adulthood; we then synthesize contemporary knowledge and approaches to assessment and assessment feedback adopting a strength-focused, person-centered, and neuro-affirming approach, in the following domains: social communication and interaction, behaviors and interests, intellectual, and executive functioning and cognitive styles, language and communication, adaptive functioning, behavior, mental health, and wellbeing. We also explore assessment during times of transition, for employment, and assessment relating to aging. In each of these domains, we discuss what to assess (i.e., what information is important to explore in the assessment) and how to assess (i.e., how to gather this information using a range of assessment approaches).