SUMMARY
Many indicator-based methods for the environmental assessment of farming systems have been developed. It is not the absolute values of the indicators that reveal whether the impact of a system is acceptable, but rather the distance between these values and some reference values. We reviewed eight frameworks for the environmental assessment of agricultural systems that define reference values for their indicators. We analyzed the methods used to establish reference values and explored how to improve these methods to increase their usage and relevance. This analysis revealed a striking diversity of terminology, sources, and modes of expression of results. Normative reference values allow the assessment of a single system with a previously defined value; Relative reference values are based on indicator values for similar systems or a reference system. Normative reference values can be Science-based or Policy-based. A science-based normative reference value can be a Target value, which identifies desirable conditions, or an Environmental limit, which is the level beyond which conditions are unacceptable. The quantification of the uncertainty of reference values is a topic which is barely explored and warrants further research. Reference values present a means of introducing site specificity into methods for environmental assessment which seems, at present, largely under-exploited.