Most of the information presented on this species account comes from studies that my former graduate students, Verónica Anadón-Irizarry and Gustavo Miguel González, and I conducted in Puerto Rico, most intensively in the Maricao Forest at the western end of the island. These studies could not have been possible without the financial support of BirdLife International and the U.S. Fish and Wildlife Service. The Department of Natural and Environmental Resources of Puerto Rico and the U.S. Forest Service granted us permits to conduct research with the Elfin-woods Warbler in public and protected lands under their jurisdiction. José Cortes, Antonio Iñigo, Ferdinand Rivera-Villalba, Carlos Diaz, Sara Madera, Harry Maldonado, and Juan C. Martínez-Cruzado graciously allowed us to conduct research on their lands. Undergraduate students, too many to be named here, from the Biology Department, University of Puerto Rico, Mayagüez campus, helped us with field work. The University of Puerto Rico, Mayagüez campus, provided us with ground transportation to travel to our research areas in Puerto Rico. Samuel González and Ramón L. Nieves, Executive Directors of Compañía de Parques Nacionales de Puerto Rico provided us with accommodations in Cabañas del Monte del Estado in Maricao Forest. The recorded song of the Elfin-woods Warbler is courtesy of José Antonio Colón-López.