Since the beginning of the development of the Santos Basin Pre-Salt fields, Petrobras has considered water and gas injection wells, whether for reservoir management purposes, alternating the well considered for the disposal of CO2-rich gas, which guarantees the reduction of greenhouse gas emissions, or for the effective application of WAG (Water Alternating Gas) method, which provides alternated injection of water and gas with the objective of increasing reservoir recovery factor. From subsea system point of view, the implementation of WAG injection at Petrobras was guided by the use of standardized XTs, and the application of 2 main configurations: satellite WAG wells and subsea manifolds for water and gas injection (MSIAG) connected to 2 wells. However, despite the advantages associated with the systems proposed for the WAG injection strategy, some additional risks ended up being attributed to the technology, such as the existence of a spec break in the XT, observed when comparing the pressure classes of the gas injection lines and water injection lines, and the increased probability of hydrate formation due to problems related to the subsea system cleaning, during fluid conversions, or gas migration through the double barrier. When evaluating MSIAG case, these risks become even worse, requiring a more rigid cleaning of the system and additional care with eventual gas migration through the double barrier of the water and gas injection system. In addition, substantial changes were also observed in the commissioning procedures and in the safety system implemented in the MSIAG, when compared to the satellite WAG wells, caused by the difficulties related to the necessary adjustments in the independence of the topside modules. Finally, the WAG Loop concept was proposed, based on connecting two WAG wells to each other and to the platform using specific pressure class lines for gas. In this proposal, it was possible to observe a reduction in the number of risers, in the CAPEX for the subsea systems, in the complexity of the safety logic and in the associated operational risk. Thus, the present work intends to present the evolution of the WAG injection concept at Petrobras from the subsea engineering point of view, as well as the operational aspects of the concepts in operation today and future perspectives for the WAG Loop.