|The effort to inventory the Hydrophilid fauna of Costa Rica is part of a larger project to inventory the aquatic beetle fauna of the country, supported by the Insituto Nacional de Biodiversidad (INBio). The goal of this larger project is to provide updated (and in many cases, the first) keys, biogeographical data, habitat preferences and other references in both English and Spanish. The initial basis for the review of the Hydrophilidae of Costa Rica comes from the existing collection at INBio that has been assembled by parataxonomists and collaborators since 1990; the holdings of the family currently number around 25,000 specimens. This information has been supplemented by a series of colleting expeditions by the principle collaborators over the past four years and parataxonomists trained to look for these groups. Tens of thousands of additional specimens from dozens of additional localities have been collected on these expeditions. The Hydrophilidae represent the most speciose family of aquatic beetles in Costa Rica, and at least several dozen more species await description.