Little is known about the structure and function of hummingbird vocalizations. We studied the vocalizations of Blue-throated Hummingbirds (Lampornis clemenciae) at two sites in southeastern Arizona. Songs were produced by males and females. Male songs consisted of arrays of notes organized in clusters of “song units.” Within sites, all males shared the same song units. Individual differences occurred in some temporal aspects of song, and slight but consistent differences in note structure occurred between the two sites. The organization of units within songs was marked by rigid syntax, and long songs were produced by agglutination of units. Male songs may function in territorial advertisement and mate attraction. Female songs were very different acoustically from those of males and typically were given when females were within a few centimeters of a male. In these situations, the female's song often overlapped temporally with the male's song. Of the hummingbird species studied so far, the Blue-throated Hummingbird has the most complex songs and is the only known species with complex female songs. Blue-throated Hummingbirds show convergence with oscines in vocal complexity, song organization, song function, and possible learning of some song elements.