Complementary and alternative medicine is widely used for children with autism spectrum disorder, despite uncertainty regarding efficacy. This review describes complementary and alternative practices commonly used among this population, the rationale for the use of each practice, as well as the side-effect profile and evidence for efficacy. The existing evidence base indicates that melatonin can be recommended as a treatment for sleeping disturbances associated with autism spectrum disorder, while secretin can be rejected as an efficacious treatment for broader autistic symptoms. There is insufficient evidence to draw conclusions on the efficacy of modified diets, hyperbaric oxygen therapy, immune therapy, and vitamin and fatty acid supplementation. There is a clear need for methodologically rigorous studies to provide evidence-based guidance to families and clinicians regarding complementary and alternative practices for individuals with autism spectrum disorders.