Autism spectrum disorders (ASDs) include a heterogeneous group of neurodevelopmental disorders with early onset in childhood. ASDs should be considered lifelong clinical entities, although there is a certain variability in developmental trajectories, and therefore should be considered of great interest also for adulthood psychiatrists. A few studies have been carried out to explore the clinical picture and course development of these disorders during adulthood, or their relationship with other mental disorders. Indeed, ASDs often share overlapping features with other disorders, such as schizophrenia and obsessive-compulsive, mood, and personality disorders, and as a result misdiagnoses often occur. The aim of this review is to summarize the available literature on ASDs in adulthood with a specific focus on the clinical picture, course, and psychiatric comorbidity. It is proposed that a careful diagnostic screening for ASDs in adults would contribute to clarifying the relationship with comorbid psychiatric disorders, while improving the possibility of treatment and outcome of such conditions.