This introduction aims at giving an overview of the questions and problems addressed jointly in Natural Language Processing and Cognitive Science. More precisely, the idea of this introduction, and more generally of this book, is to address how these elds can fertilize each other, bringing recent advances to produce richer studies. Natural Language Processing is fundamentally dealing with semantics and more generally with knowledge. Cognitive Science is also mostly dealing with knowledge: how knowledge is acquired and processed in the brain. The two domains have developed largely independently, as we will see in the introduction, but there are obvious links between the two, and a large number of researchers have investigated problems involving the two elds, either in the data or the methods used.