This chapter addresses the online variant of the container-stacking problem that the first French container port of Le Havre faces. This problem occurs when terminals are subjected to unpredictable changes and when information about the storage process cannot be available a priori to plan for containers' storage. In such cases, containers should be stored online, dynamically, and according to the terminal's current state. For that, the authors suggest making multi-criteria decisions to select container's position. They propose an online multi-levels heuristic approach that determines, at each level, where a container should be stored. They use the TOPSIS method to solve the multi-criteria decision-making problem. In order to define the value of the weights of the criteria in TOPSIS, they suggest a simulation optimization approach that can be carried out offline. The approach is used to solve the real problem of container stacking in a new intermodal terminal in the Paris region, and the results show that the approach performs better than standard online stacking rules.