Sustainable agriculture has gained acceptance as a conceptual approach for shaping farming systems of the future. All definitions of sustainable agriculture include food productivity, food safety, resource protection, quality of life and environmental quality. However, the sustainability of a wide range of farming systems has been judged only subjectively. Currently there are no scientific criteria to evaluate the sustainability of a specific farming system. We propose a framework for evaluating the relative sustainability of a farming system using nine attributes: profitability, productivity, soil quality, water quality, air quality, energy efficiency, fish and wildlife habitat, quality of life, and social acceptance. Each attribute is scored and then weighted in a way that is subjective and dependent on the judgment of the evaluating team, but that must be expressed numerically. The scoring must be based on quantifiable constraints within each attribute. Constraints can be quantified by direct measurement, which is already true for those related to profitability, productivity, water quality and energy efficiency. Constraints that are not readily measurable will need other evaluation techniques, including expert opinion and computer simulation models.