The aim of this project is to account for the diachronic development of que (‘that’)-omission in Spanish complement constructions, in interaction with developments in clause-internal marking, as well as contact-induced change. Complementizer dropping emerges in the 15th century, is most frequent in the 16th and 17th century and diminishes from the 18th century onwards, with only partial remnants in contemporary Spanish. On the basis of data from the corpus CODEA+2015 in the timespan between 1400 and 1800, the project will try to understand: I) which semantic, syntactic and pragmatic features of the main and the clausal predicate influence clause linkage without que; II) the role of participants econding in the main and the subclause for the omission of the complementizer; III) the role of factivity in the subclause. We will use both quantitative statistical methods and qualitative analysis in order to address these topics. The rise and demise of complementizer deletion will also be studied in relation to other syntactic constructions such as the accusativus cum infinitivo and the el que-clauses. This comparison with other complementation patterns will help to clarify the relationship of que-deletion with a more general process of restructuring the complementation system from Medieval Castilian to Modern Spanish. In addition, there are indications that language external factors of change (e.g. discourse traditions) may also have played a role. The omission of the complementizer is considered to originate as a Latin calque from the humanist era, but it was also quite common in letters of the lower elite in the 16th century. Therefore, the analysis of inter- and intra-author-variation may also shed new light on the locus of change, as well as an analysis in terms of the distribution of this complementation strategies in various discourse traditions and text types.