The efficiency-oriented part of the literature on informal governance points to institutional costs as a reason for governments to prefer to cooperate with each other through commitments that are not binding. Left unexplained is what I call the dilemma of informal governance: how informal governance copes with the problem of cheating, to which formal governance has traditionally provided the solution. I show that like-mindedness, the current solution to the dilemma, is convincing but underspecified. Working from a model of governance encompassing the three time-honored dimensions of obligation, precision, and delegation, I analytically explore two other solutions, one that fails, information transmission, another that works, outside option, which I borrow from the power-oriented part of the literature on informal governance. A key finding is that informal governance, despite being neither self-enforceable nor informative, is sustainable for mild Prisoner’s Dilemma (PD) types in the presence of outside options. I illustrate the model findings by tracing an historical correlation between power polarization and formalism in the design of security regimes.