SUMMARYDientamoeba fragilisis an inhabitant of the human bowel and is associated with gastrointestinal illness. Despite its discovery over a century ago, the details ofDientamoeba'slife cycle are unclear and its mode of transmission is unknown. Several theories exist which attempt to explain howDientamoebamay be transmitted. One theory suggests that animals are responsible for the transmission ofDientamoeba. However, reports ofDientamoebain animals are sporadic and most are not supported by molecular evidence. Another theory suggests thatDientamoebamay be transmitted via the ova of a helminth. Given that the closest relative ofDientamoebais transmitted via the ova of a helminth, this theory seems plausible. It has also been suggested thatDientamoebacould be transmitted directly between humans. This theory also seems plausible given that other relatives ofDientamoebaare transmitted in this way. Despite numerous investigations,Dientamoeba's mode of transmission remains unknown. This review discusses the strengths and weaknesses of theories relating toDientamoeba's mode of transmission and, by doing so, indicates where gaps in current knowledge exist. Where information is lacking, suggestions are made as to how future research could improve our knowledge on the life cycle ofDientamoeba.