This thesis describes and analyses patterns of complementation and auxiliation in the languages spoken in an understudied area of Italy, namely Southern Lazio. From a descriptive perspective, this thesis serves to document several severely endangered Romance languages spoken in the Italian peninsula. In so doing, several previously undocumented complementizer and auxiliary systems are illustrated for the first time. From a theoretical perspective, this thesis accounts for the patterns of variation found in these auxiliary and complementizer systems. Traditional descriptions of Italo- Romance treat these systems as entirely unrelated. Indeed, to date, no previous study has compared the distribution of complementizers and auxiliaries in Italo-Romance to investigate similarities and correspondences between them. This dissertation takes the original step of demonstrating that the distribution of particular auxiliary systems correlates with the distribution of particular complementizer systems, offering, in turn, an integrated and complementary theoretical analysis of both phenomena.