ASD is a heterogeneous disorder i.e. no two individuals on the spectrum will have the same set of signs and symptoms. The severity and range of symptoms are highly variable. However, the symptoms or difficulties can be classified into core domains such as difficulty in social interactions, communication deficits, behavioral issues and unusual interests and certain physical attributes. An individual may present with anyone, or a combination of or all of these difficulties/symptoms. Signs and symptoms usually become noticeable in the first three years of life. This section details the range of these signs and symptoms which are usually observed in individuals with autism.