The cosmopolitan subfamily Acidocerinae (Coleoptera: Hydrophilidae) is one of the largest and most taxonomically challenging lineages of water scavenger beetles. Recent phylogenetic studies have substantially advanced our understanding of acidocerine relationships but also illuminated the twin challenges of poorly delineated generic concepts and a classification broadly incompatible with the phylogeny. Here, these two challenges are addressed by providing a comprehensive synthesis and taxonomic tools for the Acidocerinae, including (1) a brief history and the current state of acidocerine classification, (2) a review of acidocerine ecology and collection methods, (3) the current knowledge of larval and fossil acidocerines, (4) a morphological primer on characters of taxonomic and systematic importance within the lineage, (5) a key to the world genera of Acidocerinae, (6) diagnoses, habitus, and aedeagal images, distribution maps, and summary of knowledge for each of the 23 extant genera in the subfamily, and (7) a complete annotated taxonomic catalog including the published distributions, synonyms, and references for all described 541 acidocerine species recognized as of 1 April 2021. The following nomenclatural acts are proposed to bring the phylogeny and classification into alignment:Colossocharesgen. nov.is established to accommodate two African species previously described asHelochares(s. str.);Novocharesgen. nov.is newly established to accommodate 15 Neotropical species previously included inHelochares(s. str.); the remainingHelocharessubgeneraHelocharimorphusKuwertsyn. nov.andHydrobaticusMacLeaysyn. nov.are synonymized withHelocharesMulsant.PeltocharesRégimbartsensu nov.is redefined to include eight Old World species previously included inHelochares(s. str.). A lectotype is designated forPeltochares conspicuusRégimbart, the type species of the genus. The taxonomic and morphological circumscription ofHelocharessensu nov.is narrowed and redefined.
