The long-held belief that the fetus is sterile prior to birth has been questioned recently by a plethora of studies, using highly-sensitive molecular sequencing techniques, reporting the presence of bacterial and viral DNA within feto-placental tissues and fluids. Contradictory studies have also appeared, refuting the claims that a fetal microbiome exists and concluding that the results are confounded by the difficulties in properly accounting for external contamination. The limitations and interpretations of the conflicting evidence have elicited robust debate amongst the research community. The proponents of both sides place their findings in the context of the potential impact of microbial exposure on the development of the early-life immune system and postnatal microbiome; they argue that the sterility (or otherwise) of the fetal environment in normal, healthy pregnancies has important health implications. Yet, although there is growing evidence linking the development of the post-natal microbiome with risk of a range of disorders and diseases in later life, the health and developmental consequences of antenatal microbial exposure remain the matter of conjecture. In this presentation I will attempt to evaluate and summarise the evidence and controversies around the existence and nature of the “antenatal microbiome” and its microbiological and immunological significance for fetal, pediatric and adult health.