There is an emerging body of evidence linking the intestinal microbiota with autism spectrum disorders (ASD). Studies have demonstrated differences in the composition of gut bacteria between children with ASD and controls. Certain intestinal bacteria have been observed in abundance and may be involved in the pathogenesis of ASD; including members of the Clostridium and Sutterella genus. Evidence from animal models suggest that certain microbial shifts in the gut may produce changes consistent with the clinical picture of autism, with proposed mechanisms including toxin production, aberrations in fermentation processes/products, and immunological and metabolic abnormalities. In this article, we review studies examining the relationship between intestinal bacteria and ASD, and discuss bacterial species that may be implicated and proposed mechanisms.