The taxonomic history of Culex pipiens (1758–present) is reviewed. The central question is whether Cx. pipiens is a single polytypic species or a complex of sibling species? The taxon traditionally known as the Cx. pipiens complex is referred to as the Pipiens Assemblage to avoid difficulties associated with the meaning of the word “complex”. Neotype specimens have been designated to fix the morphological identities of Cx. pipiens, Cx. molestus, and Cx. quinquefasciatus, and Cx. pallens is represented by a holotype, but whether these nominal forms represent one or more biological species remains controversial. Despite extensive morphological and physiological/behavioral variation, there is no indication of subspecific or racial differences in geographically separated populations of Cx. pipiens. Introgression occurs where populations of Cx. pipiens and Cx. quinquefasciatus overlap, but the retention of parental epiphenotypes outside the zone of introgression provides evidence of independent species cohesion. The main conclusions reached are: Cx. pipiens and Cx. quinquefasciatus are separate species which evolved in Africa and hybridize in non-indigenous areas where they were unintentionally introduced by humans; Cx. molestus is nothing more than a phenotypic and physiological variant of Cx. pipiens; and Cx. pallens has no taxonomic status under the provisions of the International Code of Zoological Nomenclature. Based on morphological similarity, the Pipiens Assemblage includes Cx. pipiens, Cx. quinquefasciatus, and possibly Cx. australicus. There is no evidence to suggest that the Pipiens Assemblage includes any other species.