Autism spectrum disorders (ASD) are a group of related neurodevelopmental disorders, which includes autism (autistic disorder), Asperger syndrome, Rett syndrome, pervasive developmental disorder-not-otherwise specified (PDD-NOS), and childhood disintegrative disorder (CDD). This chapter provides the review of recent knowledge about clinical symptoms and criteria for diagnosis of heterogeneous symptoms of ASD. An alarming increase in the prevalence of ASD is of great concern to practicing pediatricians and psychiatrists. Some people attribute the increases over time in the frequency of ASD to factors such as new administrative classifications, changing diagnostic criteria, and heightened awareness. It is evident, that no single factor or a simple explanation can account for the increase. ASD are highly genetic and multifactorial, with many risk factors acting together. There is no therapy of the core symptoms of ASD at present. Several studies suggest that 50-75% of children with ASD are using complementary alternative medicine. Families and clinicians need access to theoretical and clinical evidence to assist them in the choice of therapies.