Metastrongyles are common lung worms of mammals including most aquatic species. Given the importance of the respiratory system for diving and food acquisition in marine mammals, metastrongyles could be particularly detrimental in this mammalian group. However, there is conflicting preliminary evidence regarding the health consequences of metastrongyle infections in marine mammals and little understanding on the factors that could drive pathogenic effects. To fill this gap, we conducted a systematic review of the literature to analyze the diversity of this parasite group in marine mammals and the host and parasite factors that could be associated with pathogenic effects. There have been at least 3 families and 40 species of metastrongyles described in 2 orders, 10 families, and 66 species of marine mammals. The Indo-pacific finless porpoise, narrow-ridge finless porpoise, and short-beaked common dolphin harbored the highest diversity of metastrongyle species whileHalocercus hyperoodoni,Otostrongylus circumlitus,Parafilaroides gymnurus,Halocercus brasiliensis, andStenurus minorwere the metastrongyles which parasitized more marine mammal species. Metastrongyle infections are most frequently documented in the lower respiratory system, otic apparatus, and cardiovascular system of marine mammals. For host-parasite relationships where metastrongyles were found in the respiratory system and pathogenic effect was assessed (n=166), respiratory specific pathogenicity such as bronchopneumonia was reported in 80.12% of the cases (n=133). In contrast, for relationships where metastrongyles were found in the otic apparatus and pathogenic effect was assessed (n=26), otic specific pathogenicity such as otitis was reported in only 23.08% of the cases (n=6). Similarly, for relationships where metastrongyles were found in the cardiovascular system and pathogenic effect was assessed (n=11), cardiovascular specific pathogenicity such as vasculitis was reported in 45.45% of the cases (n=5). Finally, metastrongyle species that infected a larger number of marine mammal species of larger size were more likely to induce pathogenic effects on the host such as tissue specific inflammation and mortality. Metastrongyles are important pathogens of marine mammals that can cause significant tissue damage and mortality although pathogenic host-parasite relationships are dominated by a few metastrongyle species with wider host ranges.