Mendelian inheritance transfers genes vertically within lineages, whereas horizontal gene transfer (HGT) moves genetic material between or among lineages. Herein, we explore possible mechanisms of HGT between parasites and their hosts, as their intimate contact affords substantial opportunities for HGT. We review studies of host-parasite HGT, discussing their merits, their shortcomings, and the multiple lines of evidence needed to conclusively document HGT while avoiding false positives. We focus mainly on schistosomes and other parasites with complex life cycles as they provide numerous opportunities for HGT among the parasite, intermediate, and definitive host genomes. We also highlight future research directions that could prove illuminating with regard to the occurrence, prevalence, and overall importance of HGT in host-parasite coevolution.