International greenhouse gas offset credits from developing countries could play a major role in fulfilling developed countries’ emission reduction pledges under the Cancún Agreements, but there is great uncertainty about the future role of such offsets. This paper focuses on a key question: whether both the developing countries generating the offsets and the developed countries buying them will be allowed to count the same emission reductions toward their respective pledges. The authors quantify the implications of double-counting of international offsets by building a spreadsheet model to analyze how potential offset supply and demand balances may evolve. They find that double-counting could effectively reduce the ambition of current pledges by up to 1.6 billion tons CO2e in 2020. They close by describing several possible ways to address the risks of offset double-counting.