Autism Spectrum Disorder (ASD) is a type of neuropsychological disorder that targets cognitive development and is often the cause of repetitive behaviors and social disabilities. In the 21st century, multiple research projects and studies have been conducted relating to this disorder. New ideas were published, and more developed theories corresponding to ASD were suggested. This paper reviews the schemes and ideas proposed about ASD in this century using a literature review method and is based on three main aspects: etiology, symptoms, and treatments. Firstly, this paper will elaborate on the various causes of ASD, including genetic, sexual, and parental factors, using recent studies done by professionals. Then, this paper will introduce the numerous symptoms that can be caused by ASD: cognitive and social deficits. Each of these phenomena will be discussed with studies and experiments. Finally, this paper describes how different types of treatments are used to reduce the symptoms of ASD, including applied behavior analysis (ABA), pharmaceutical treatments, and assistive technology (AT). Each of the corresponding treatments is discussed based on their advantages and disadvantages. In conclusion, current research on ASD has yielded some results, while at the same time this area of research requires further development in order to clarify controversial or unclear issues.