with the ideas of avoiding wasteful consumption, owning fewer products, and prolonging their lifetimes. Here also, depending on authors’ definitions, the boundary between efficiency and sufficiency may not always be perfectly drawn. As an illustration, a report for UNEP classifies items such as reducing living spaces, driving smaller cars and car sharing as ‘material efficiency’, while they would be more traditionally viewed as sufficiency. Potential impact Sufficiency largely remains a blindspot in most established ecological/energy transition scenarios, where efficiency and greener technologies are the main and only strategies usually modelled. There are exceptions though. The French négaWatt Association has assessed the potential of energy sufficiency at the level of France, through its national négaWatt 2050 scenario. The scenario is based on three principles (sufficiency-efficiency-renewables) with the goal of reaching a factor 2 reduction on energy demand and factor 4 on greenhouse gas emissions by 2050, involving changes in lifestyles and societal organisation considered as reasonable by the modellers. Sufficiency appears to be able to provide more final energy savings than efficiency. A German study found that sufficiency measures could lead to a theoretical reduction of 70 % of energy consumption at the level of households. The calculation assumes sufficiency measures encompassing the size of appliances (smaller, fewer appliances) as well as in their usage patterns. A number of other scenarios and models lead to similar conclusions on the magnitude of saving potentials. In order to encourage and improve the robustness and visibility of sufficiency modelling in energy scenarios, guidelines and recommendations have been published e.g. by the Ministry of Environment in Germany, or the SHIFT Project in France). Implementation Principles While several established environmental strategies and policies do not question upstream the need for perpetual growth in energy and material services, sufficiency does. It needs to be translated into implementation principles to challenge current lifestyles and social paradigms. The "Four D's" suggested by Wolfgang Sachs are one example: Decelerate (going slower and less far); De-clutter (accumulate fewer things); Decentralize (choosing local and regional) ; Decommercialization (leaving less room for the market). The discussion about sufficiency principles is not restricted to a particular area or sector, and may relate to broad lifestyle aspects such as quality of life and work-life balance. Sufficiency, as a strategy, may also be operationalised through distinguishing between different approaches to limiting the need for energy/resource-intensive services: Reducing (i.e. consuming less); Substituting (replacing highly-consuming services by less