Serotonin, a neurotransmitter found throughout the brain and body, has long been of interest in autism. Repeated findings of elevated platelet serotonin levels in approximately one third of children with autism has led some to believe that dysfunctional serotonin signaling may be a causal mechanism for the disorder. Because serotonin is critical to fetal brain development, concerns have arisen regarding prenatal exposure to substances that manipulate serotonin levels, such as selective serotonin reuptake inhibitors (SSRIs). This review examines evidence regarding the serotonin system and autism spectrum disorders (ASD), as well as what the literature has reported thus far on developmental effects of prenatal exposure to SSRIs. Possible mechanisms by which SSRIs could affect the fetus during pregnancy and clinical implications are also discussed. Though the majority of studies conducted in infants and children suggest prenatal exposure to SSRIs does not affect neurodevelopment, interpretation must be tempered given small sample sizes. The only published study that focused on prenatal SSRI exposure and ASD found an increased risk with exposure to SSRIs, especially during the first trimester. Obstacles that will be faced in future research are isolating medication effects from maternal depression and, given the infrequent occurrence of exposure and outcome, obtaining an adequate sample size. Whether serotonin is an etiologic factor in ASD, and what it points to as a marker for subgrouping, remains unclear. Understanding how the development of ASD might be affected by prenatal factors that influence serotonin levels, such as SSRIs, could identify modifiable targets for prevention. Autism Res 2013, 6: 149–168. © 2013 International Society for Autism Research, Wiley Periodicals, Inc.