This paper explores the implications of setting a minimum ratification level on an international agreement to tackle the Greenhouse Effect. Several aspects of the ratification level are considered, including the threshold number of signatories required to effect agreement, the potential number of participating countries, and the distribution of benefits from taking action. The likelihood of reaching agreement on a ratification level is also considered. It is shown, for example, that the optimal ratification level is reasonably robust to variations in circumstances, and that the prospects for effecting a treaty may be improved by there being a large number of countries. This paper focuses on the implications of one particular aspect of reaching agreement on tackling the Greenhouse Effect: that concerning the minimum number of countries that are required to commit themselves to participate before action is taken by any.' While representing only one dimension of the multi-faceted design of an action plan for overcoming this potentially grave problem, the setting of a required threshold ratification level could be an important element in concluding an agreement. Several aspects of a minimum ratification level are explored. For example, we consider the consequences of varying the number of participants required to ratify an atmospheric treaty. We also examine how the expected global gain from a treaty depends on the potential number of signatories, and on their costs and benefits of compliance. It is shown, for example, that, within the specific environment of analysis, inclusion of a minimum agreement level, or 'n-rule', is distinctly better than allowing independent action. However, it is also shown that uncertainty over different countries' evaluations of the net benefits of action is a potentially serious difficulty. Specifically, even in a situation in which the threshold number of participants can be optimally set and in which credible threats can be made not to repeat the opportunity to cooperate, the expected benefit of the minimum pre-commitment approach may be well below that which would be achieved if all countries could be forced to cooperate.2 Nevertheless, it is also shown that the global nature of the problem, specifically the large number of countries involved, has a positive influence on what can be achieved by having a ratification threshold, or 'n-rule'. This is a less than intuitive outcome.3 Furthermore, it is shown that, with