Oomycetes, or water moulds, are fungal-like organisms phylogenetically related to algae. They cause devastating diseases to both plants and animals. Here, we describe seven oomycete species that are emerging or re-emerging threats to agriculture, horticulture, aquaculture, and natural ecosystems. They include the plant pathogens Phytophthora infestans, Phytophthora palmivora, Phytophthora ramorum, Plasmopara obducens, and the animal pathogens Aphanomyces invadans, Saprolegnia parasitica, and Halioticida noduliformans. For each species, we describe its pathology, importance, and impact, discuss why it is an emerging threat, and briefly review current research activities.