This article aims at describing formal and functional properties of Romance complementizers. We provide an overview of Romance languages and propose functional and formal definitions of complementation, complementizers and ‘canonical’ complementizers, focusing on the Romance complementizers corresponding to the English if and that. We describe the syntactic behavior of these complementizers, then focus on their semantic aspects and distribution; we show that a general function of illocutionary subordination is common to all complementizers, as is proven by the semantic effects of complementizer omission. The semantic analysis of these complementizers enables us to specify which kind of non-factuality is encoded in complements introduced by ‘if’ complementizers. We go on to examine the diachronic sources of Romance complementizers and we finally focus on the marginal deviations found in Romanian on the one hand and French on the other hand as compared to other Romance languages.