Nowadays, innovation is getting an ever-increasing importance as competition and demand acquire higher complexity. For this reason, R&D projects are crucially important for the success of companies. It is common that very few projects are selected among the initial proposals, because of time and resources constraints, and thus it is important for companies to have an established selection method to prioritize each proposal. Project selection is thus an essential process and is the focus of this thesis. Project Portfolio Selection has been extensively studied in the literature, defining plenty of approaches and methods to handle this issue. In this work, particular emphasis is put on the models that consider multiple criteria for the prioritization and selection of projects, and two among the most popular ones, AHP and PROMETHEE, will be studied in detail.