The Cossidae is an economically important family consisting of six subfamilies and more than 700 species globally. Their larvae are often of serious concern to forestry and horticulture in that they are known to bore in the branches and trunks of a wide range of trees and shrubs. In the course of my examination of the extensive collections of the Smithsonian Institution and the Instituto Nacional de Biodiversidad of Costa Rica, I have found the Costa Rican Cossulinae to consist of 17 species, including 6 new to science. The phylogenetic relationships of all cossuline genera, including 2 new genera, have been analyzed using the computer programs PAUP, WINCLADA and NONA. Data for the study has been derived from extensive microscopic slide preparations and examinations of both external (body, leg, and wing morphology) and internal (primarily genitalic morphology) anatomical structures. Diagnostic characters of all taxa have been photographed using digital equipment involving automontage, supplemented with camera lucida ink drawings for additional detail where necessary. Diagnostic descriptions and identification keys accompanied by detailed illustrations of every species are being generated, and the recorded distribution of each species will be clearly displayed using topographic maps of Costa Rica.