All major climate policy agreements - the UN Framework Convention, the Kyoto Protocol and recently the Copenhagen Accord - have stated that climate finance for developing countries will be ”new and additional”. However, the term “new and additional” has never been properly defined. Agreeing a system to measure a baseline from which “new and additional” funding will be calculated will be central to building trust and realising any post-Kyoto agreement. We explore eight different options for a baseline, and assess each according to several criteria: novelty to existing pledges, additionality to development assistance, environmental effectiveness, distributional consequences, and institutional and political feasibility. Only two baseline options do well on these criteria and are therefore viable: "new funds only" and "above pre-defined business as usual level of development assistance". The final section assesses the impact of the baseline definition on the novelty and additionality of “fast start finance” pledged under the 2009 Copenhagen Accord, showing that values can vary from 0 to 100% depending on the definition.