This introduction of the special issue on Transitivity and Valency aims to a rapid state-of-the art retrospective examination of the topic and to the presentation of the key points of focus of the contributions included. Two papers discuss valency from a theoretical perspective: the contributions present (a) a new linguistic model, namely the bi-directional version of Superoptimality, within the framework of Optimality Theory (Legendre and Smolensky) and (b) a novel formal structure, namely polygraphs (Mazziotta). Two papers include diachronic data that provide evidence for valency change, juxtaposing Old and Modern French (Mazziotta) or Medieval French (MF) and other Medieval Romance varieties and their diachronic development (Wolfsgruber). We also find a contribution that adheres in the typological literature, examining a member of the Andic group of languages (Creissels). Finally, there is evidence from the field of first language acquisition (PĂ©rez-Leroux, Pirvulescu, Roberge, & Strik) and an attempt to link the acquisitional aspect of the issue to historical linguistics (Wolfsgruber).