Autism Spectrum Disorders (ASDs) consist a group of neurodevelopmental disorders that are usually diagnosed in early childhood but they persist throughout life, although significant changes can happen. The prevalence of the ASDs is estimated to be 1-1.2%. Subjects with the more severe form of the disorder that are usually characterised by the absence of a communicative language and learning difficulties of various severity, are often referred as persons with lower functioning. In the other end of the spectrum we can find subjects with less severe symptomatology, communicative language and at least of normal intelligence that are referred as high functioning autistic people or -in case of an absence of a language delay- as suffering from Asperger syndrome. The lower functioning adults can be referred to an adult psychiatrist mainly due to their behavioral problems and disruptive behaviors. Their inability to express their difficulties, due to their language restrictions and empathy deficits, can lead these people to behavioural deviances (often self- or hetero-destructive) that challenge their personal environment ending up in the pursuit of psychiatric help. In most cases, although not always justified, psychotropic medications will be prescribed in an attempt to control their maladaptive behaviors. Special attention should be paid to the catatonic exacerbation of ASD, which can be exhibited after adolescence. The catatonic features presented shouldn't be perceived as a possible comorbidity with another disorder, such as schizophrenia, but rather as an extreme form anxiety within the context of an ASD. High Functioning adults with ASDs are more difficult to be detected, but they may also need psychiatric consultation. These subjects may have never been diagnosed with an ASD, but they could have in their history a variety of diagnostic categorizations. Their accurate diagnosis could be further hampered in cases where they are exhibiting remarkable abilities, professional success or even an adequate social adaptation, such as marriage and family. Very often their symptoms will be confused with those of other disorders and they will be also prescribed psychotropic medication with very few, if any, results. In the current paper, we will point out the symptoms and situations that should alert the psychiatrist for the presence of an ASD in an adult with a normal intelligence and adequate functioning that is referred to him for bizarre ideas or behaviors. The designated diagnostic procedure for the ascertainment of the ASD in this case is similar to the one followed for children and adolescents and comprises of a detailed developmental history and a relevant observation and interview. Finally, we will discuss the most common difficulties in the differential diagnosis of the high functioning adults with an ASD from those suffering from Obsessive Compulsive Disorder, Schizoid Personality Disorder, Schizophrenia and Psychosis, and we will provide key issues that can be of an assistance in the more accurate assessment and categorization of the presented symptoms.