This thesis studies concessions and Public-Private Partnerships (PPPs) carried out in Brazil. To analyze the problems, risks, uncertainties, and factors that affect infrastructure projects and proposes a strategy to solve one of the main risks that affect projects: demand risk. In the first part, a systematic literature review on public infrastructure projects and the application of evaluation approaches that incorporate flexibility is carried out. This is the case with the Real Options Approach. This review focuses on the main advances made in the area, current trends, ways to incorporate flexibility, and its evaluation. The second part analyses the main characteristics of concession contracts in Brazil and the factors that generated problems in concessions. This analysis identifies the main problems, similarities, and differences between sectors and displays the strategies used for facing the difficulties. The third part shows trends regarding PPPs in Latin America and the Caribbean. It also highlights a concession model used in ports. Unlike other models, construction is carried out according to demand through triggers. Once the trigger is activated, the investment becomes mandatory. Therefore, it is known as a mandatory option. Finally, a new model is developed in the fourth part to mitigate demand risk and not generate contingent liabilities for the granting authority. This new strategy is called the MRG/GL mechanism, as it combines the Minimum Revenue Guarantee (MRG) with the Accounts Mechanism. The results show that it is possible to reduce demand risk, make projects attractive, and simultaneously reduce the possibility of budgetary burden for the government.