This paper focuses on the economic desirability of the fixed and relatively short-term greenhouse gas targets and timetables in the Kyoto Protocol. The Protocol provides flexibility in which greenhouse gases to control, where control can be implemented, and what domestic policy measures are used. However, the Protocol does not allow much flexibility in when emission reductions take place in pursuit of longer-term environmental goals. Nor does it allow more flexible shorter-term environmental targets through price-based policy instruments that balance environmental goals and compliance costs. The relative inflexibility of the Protocol with respect to these elements may derive, in part, from a misplaced analogy between the global warming issue and the highly successful effort to phase out CFCs under the Montreal Protocol. The lack of "when" flexibility may be a key barrier to achieving the broader goals of the Kyoto Protocol, particularly if "where" flexibility is constrained in implementing the Protocol.