Implementation of some of the articles of the Kyoto Protocol will require rules for accounting and for defining baselines against which reduction of greenhouse-gas emissions, or enhancement of greenhouse-gas removals, are to be measured. Project accounting needs to provide incentives to ensure that the objective of the United Nations Framework Convention on Climate Change (UNFCCC) is served and that the interests of all participating parties are respected. To establish the emission reduction achievements of activities is complex as it is inherently very difficult to define the counterfatctuel baseline. Here, we articulate four basic principles — accuracy, comprehensiveness, conservativeness and practicability — that can be used to guide the construction of baselines for greenhouse-gas mitigation projects. The overall aim is to have accurate, comprehensive, and conservative baselines; but this aim needs to be balanced to yield baselines that are as simple as possible, can be practically implemented, and provide incentives to fulfill the ultimate objective of the UNFCCC.