We review outcome studies regarding the effectiveness of hyperbaric oxygen therapy for Autism Spectrum Disorders. Studies were identified through electronic bibliographic databases and manual searches of article reference lists.A total of 8 studies met eligibility criteria, consisting of three randomized controlled trials, one quasi-experimental study involving a comparison group, two pre-experimental one-group pretest–posttest studies, and two single-system designs. Studies reviewed did not offer credible evidence to suggest that HBOT is an effective treatment for autism. It is premature to call HBOT an effective treatment for Autism and ASD. Individuals clinically treated with HBOT outside the context of a RCT should have the effects of the therapy evaluated using rigorous single-subject designs.