Different types of environmental indicators have been developed to meet the increasing need of assessment instruments of environmental impacts. As any tool developed by research, indicators must be elaborated according to a scientific approach. One of the important steps of this elaboration is the validation. The overall objective of this paper is to present a methodological framework to validate indicators. According to the definition of an indicator, three kinds of validation are presented: the “design validation” to evaluate if the indicators are scientifically founded; the “output validation” to assess the soundness of the indicator outputs, and, the “end use validation” to be sure the indicator is useful and used as a decision aid tool. The output validation is inspired from validation of simulation models, which is shortly reviewed. Because indicators differ from models in many cases, validation procedures commonly used in modelling have to be adapted