Among a family renowned for its complex identification and taxonomic problems, the genus Tolmomyias represents one of its pinnacles, and the Yellow-margined Flycatcher is no exception to this rule. Distributed widely, from eastern Costa Rica south to northwest Ecuador, and then over the greater part of Amazonia, some eight subspecies are generally recognized. Subtle variation in crown, breast, and belly coloration is observable, and rather more obvious differences in vocalizations, all of which suggest that more than one species is probably involved within this complex. Indeed, some authors have already chosen to split trans-Andean and cis-Andean taxa into two species, with the Amazonian birds being named Zimmer’s Flatbill (Tolmomyias assimilis), and the Central American and northwest South American population retaining the name of Yellow-margined Flycatcher, but henceforth taking the scientific name Tolmomyias flavotectus. However, given the variation that can be observed within Amazonian birds, this approach appears far too simplistic, leaving most authorities waiting for a more robust taxonomy to become available before revising the species’ systematics.