Achieving consensus among scientists is often a challenge?particularly in model development. In this article we describe a recent scientific consensus-building process for Life Cycle Impact Assessment (LCIA) models applied to chemical emissions--including the strategy, execution, and results of a process that used model comparison to achieve parsimony. This process has succeeded in establishing a transparent LCIA consensus model. We present the lessons that may be adapted by similar consensus processes in other fields. LCIA characterizes potential impacts on human health and the environment attributable to chemical emissions over the life cycle of a product. LCIA relies on substance-specific characterization factors (CFs) that combine exposure potential and toxicity to represent the relative contribution of the substance to health and environmental impacts (1). LCIA focuses on comparative assessment, using approaches adapted from risk assessment. In 2003, in response to large variations in available methods, an international model comparison/consensus process was initiated. This process was under the umbrella of the Life Cycle Initiative, a joint effort of the United Nations Environment Program (UNEP) and the Society of Environmental Toxicology and Chemistry (SETAC) (2). The process encompassed an international group of model developers responsible for the most commonly-used worldwide LCIA characterization models and focused on characterization of human and ecosystem health impacts. It also involved disciplinary experts in fate and transport, exposure assessment, health risk assessment, and ecotoxicology. The comparison/consensus process fostered a common understanding among the participants of which model elements contribute most to the relative magnitude of LCIA characterization factors. It became clear that with a careful focus on the most influential model elements a consensus model could be established. Experience dictated that a more transparent model would be more likely to gain and retain acceptance and wide-spread use. The need for consistent documentation and transparency led the participants to create an entirely new model, building on contributions from the existing models. This required consensus on essential model elements, provided robust results consistent with existing models, and made parsimony a guiding principle. The tangible outcome is "USEtox", named in recognition of the UNEP-SETAC Life Cycle Initiative under which it was developed. The model is supported by all participating model teams as a basis for future global recommendations of LCIA characterization factors.