Besides their core symptomatology, patients with autism spectrum disorders (ASDs) often show various comorbidities. Among them, gastrointestinal (GI) dysfunction is frequently observed. Although considerable progress has been made to identify the underlying causes and pathomechanisms of ASD, it is still unclear if, and how, GI problems contribute to the etiology of ASD or influence the presence and severity of one, or multiple, of the core features. This chapter summarizes the current findings on the impact of abnormal gut-brain signaling in ASD, which has been mainly associated with altered microbiota composition and proinflammatory processes. The use of specific alterations in the microbiome reported in human and rodent studies as a biomarker for ASD will be discussed, as well as potential treatment strategies for ASD that are based on addressing components of the gut-brain axis.