The notion of auxiliary verbs and their role is discussed within Z. S. Harris' transformational framework. Following Maurice Gross' proposals, the traditional approach to the subject, usually limited to verbal combinations, is extended to adjectival and nominal constructions. Such an approach to auxiliaries leads one to generalize the notion of auxiliary verb and to propose that aspectual verbs should have a syntactic status identical to that of tense auxiliaries; furthermore, it allows a coherent description of the aspectual verbs complementation. The empirical basis of the analysis is constituted mainly by data from Portuguese but references to other languages are also made.