This paper examines the question of the functional status of auxiliaries in English. Two approaches are contrasted: one which treats auxiliaries as mere feature-carriers, and one which treats them as argument-taking predicates. It is argued that no analysis is correct for all auxiliaries: supportive DO, perfective BE, and the modals WILL (SHALL) and WOULD are argued to be feature-carriers, while progressive BE and the rest of the modals are argument-taking predicates. It is also argued that the selection of the past participle by perfective HAVE is a case of c-structure selection, not f-structure selection or realizational morphology. An account is offered for the relative ordering of HAVE and BE.