Often small and inconspicuous, parasites are both abundant and taxonomically diverse, having profound effects on ecological and evolutionary processes. Parasites are ubiquitous across land, sea, and freshwater ecosystems, where they contribute greatly to the total biodiversity and biomass. Through evolutionary time, parasites have played a major role in shaping the life history, behavior, and morphological traits of life on Earth. In ecological time, parasites can dominate food webs and ecosystem processes, through strong effects on their hosts. It is also clear that parasite effects—including disease outbreaks and mass mortality events—respond to both environmental and human-induced changes, impacting human society through effects on fisheries, agriculture, ecosystem services, and human health. Thus parasites play key roles in the structure and function of biological communities, and SERC research seeks to advance our understanding of parasite diversity, ecology, and disease dynamics.