The four extant species of sirenians are unique among marine mammals because of their herbivorous diet, exclusively warm water distribution, and separate evolutionary lineage. Past ecotoxicologic studies focused largely on determining the concentrations of toxic elements and organohalogens in tissues of dugongs and West Indian manatees. Low concentrations of lipophilic contaminants were found, but some metals present in aquatic plants and sediments were elevated. Unusual concentrations of PCDD/Fs in dugongs were likely also taken up from plants and sediments. Conclusive ecotoxicologic studies determined that biotoxins from Karenia brevis caused multiple mortality events of Florida manatees. Although more sophisticated and comprehensive analyses have been carried out in the present century, with the exception of the red tide studies, ecotoxicologic research involving sirenians has lagged behind those of other marine mammals, and no ecotoxicologic studies have been conducted on Amazonian and West African manatees. We provide suggestions for future research directions.