The choice of the mode of realization of large public investment projects, including large infrastructure projects, between the conventional mode (PSC - public sector comparator) and the PPP mode (public-private partnership), involves two major groups of challenges and pitfalls: the evaluation of the investments themselves and the evaluation of "contracts" or more generally of the "governance" PSC vs. PPP in imperfect and incomplete information contexts. Many reports reveal a serious misunderstanding of the economic analysis of the value of a public project.