As our appreciation of the diversity within the flatworms has grown, so too has our curiosity about the ways in which these varied creatures are related to one another. In particular, the parasitic groups (trematodes, cestodes and monogeneans have been the focus of enquiry. Until recently, morphology, anatomy and life histories have provided the raw data for building hypotheses on relationships. Now, ultrastructural evidence, and most recently, molecular data from nucleic acid sequences, have been brought to bear on the topic. Here, David Blair, Andrés Campos, Michael Cummings and Juan Pedro Laclette discuss the ways in which molecular data, in particular, are helping us recognize the various lineages of flatworms.