What are the fundamental behaviour and ability characteristics that are examined in a diagnostic assessment for Asperger syndrome? This paper describes those characteristics using the framework of the new DSM-5 diagnostic criteria and also describes the additional characteristics recognised by clinicians as well as variations in expression according to age and gender. The person with Asperger syndrome has the classical features of autism without intellectual disability or impairment of formal language abilities. The features include deficits in social communication and interaction, in non-verbal communication and in understanding and maintaining relationships, together with insistence on sameness, inflexible adherence to routines or ritualised behaviour and highly restricted, fixated interests. In some individuals, the social deficits can lead to frustration and anger or to social withdrawal. Although the difficulties in social interaction should not be underestimated, the ability to focus on the area of interest, coupled with intellectual ability that is either within the normal range or above average, has produced some of the most highly-achieving individuals in society.