We are grateful to Christopher Davidson, Sharon Christoph, and the Blue Moon Fund for their support of this project. We thank Adrian Forsyth for the role he has played in our plant diversity studies and for his tireless efforts to conserve the natural habitats of the Osa Peninsula. Reinaldo Aguilar and Catherine Bainbridge appreciate the time that David Roubik of the Smithsonian Tropical Research Institute has dedicated to identifying the bees they have documented as pollinators of Osa plants and Isidro Chacon-G, Costa Rica’s leading authority in Lepidoptera and Curator of the Lepidoptera collections at the Instituto Nacional de Biodiversidad (INBio), for his determinations of butterflies and moths. We thank Pedro Acevedo, William R. Anderson, Cornelis Berg, Douglas Daly, Piero Delprete, Roy H. J. Erkens, Mike Grayum, Barry Hammel, Maria L. Kawasaki, Ricardo Kriebel, David H. Lorence, Paul Maas, J. Francisco Morales, Michael Nee, Claes Persson, Daniel Santa Maria, and Nelson Zamora for their help with the identification of plant collections from the Osa. We are grateful to Robin Foster and Nancy Hensold for assistance with entering data from collections from the Osa Peninsula archived in the herbarium of the Field Museum (F).