The past decade has been witness to a huge explosion of interest in the computational aspects of game theory. One topic that has received much attention is that of mechanism design. Crudely, mechanism design can be understood as the problem of designing games so that, if every player in the game acts rationally, certain desirable outcomes will result. In mechanism design, it is usually assumed that the designer of the mechanism has complete freedom to design a mechanism as desired. But this is not the reality of most real-world mechanism design problems: when a Government develops a new law, for example, they do not usually have a blank slate, but must work start from the framework of society as it exists. In this talk, I will present work we have done on the computational aspects of such “mechanism design for legacy systems”. In the settings we consider, a principal external to a system must try to engineer a mechanism to influence the system so that certain desirable outcomes will result from the rational action action of agents within the system. We consider two ways in which a principal might influence a system: First, through communication, which can be used to modify the beliefs of system participants, upon which players will choose actions to perform; and second, by imposing taxation schemes upon the system, so that the preferences of participants are perturbed appropriately.