The definition of resource efficiency first starts with the definition of resources. So what are resources? I want to clarify here that we focus on the environmental dimension of resources, so other resources that could be considered in another context like labour, capital or time are not part of this discussion. In the environmental dimension, there are two ways of looking at resources: in the broad sense and in the strict sense. The broad sense considers resources as “inputs” into a system but also the environment itself as a sink and accounts for its role in absorbing emissions. Resources defined in the strict sense only consider “inputs” entering an anthropogenic system, so for example materials consumed by a city or water consumed at an industrial plant. The broad sense is primarily used in a policy context and the strict sense is mainly used in industry and engineering, as resource consumption is the starting point for all economic production and consumption activities. So as a developer of a new circular product, following the strict definition of resources makes more sense. Of course, even if it does not fall into the term of resource efficiency evaluation, the evaluation of the impact of emissions on the environment should not be discarded. The coupling of resource efficiency evaluation with emission-based evaluation such as risk assessment and life cycle assessment should still be pursued. Even when following the strict sense of resources, several approaches that consider several types of resources are still possible. For example some approaches consider that resources are only raw materials while others define energy carriers such as electricity and heat as resources. Some even define resources as objects from nature, which then excludes waste that could be used as a resource in circular systems. Some definitions also include waste as a resource as well as non-tangible energy carriers like solar or wind energy, they define resources as energy, raw materials and water. One major resource, which is becoming more and more scarce around the world is land, which should also be added in such a definition. To conclude on the definition of resources, considering land, energy, primary and secondary raw materials and water allows having a complete vision of the resource use of new circular systems. Now that we have clarified what resources are, let’s define resource efficiency. In general, efficiency is defined as the ratio between the benefits obtained from a process or system, and the efforts put into this process or system. Some national and intranational governments use such a ratio to follow the resource efficiency of their region. For example, the indicator defined by the European Commission to measure the resource efficiency of Europe uses this ratio, as it is the ratio of the benefits obtained by Europeans from the use of resources, the Gross Domestic Product over Domestic Material Consumption, which is the amount of resources used. The calculation of the resource efficiency of your system will also depend on the level you choose to make the calculation. Resource efficiency can be calculated at different levels, from a single process unit to a production plant, an industrial sector or a country. It is called the foreground system and is often “controlled” by the persons in charge of the study. It consumes both resources directly extracted from the natural environment, for example water from surrounding water bodies, and processed natural resources such as electricity, and it delivers products and services to end users. The resource efficiency ratio is then calculated as the ratio of the benefits obtained from resources, in green, and the amount of resources consumed by the foreground system, in red. This approach is called the gate-to-gate approach, and typically, you can use it to calculate the resource efficiency of your production process, for example the recycling process of metals recovered from waste to obtain new materials. Another approach consists in following a life cycle perspective. In this case, the denominator of the resource efficiency ratio will be the amount of natural resources consumed along the whole life cycle of your product. Typically, an LCA will be conducted to calculate the denominator. The resource efficiency ratio then becomes the ratio of the benefits obtained from resources over the amount of resources directly and indirectly consumed from the natural environment and from waste produced by the economy. While the first approach quantifies the denominator by accounting for the amount of resources used by your main production process, the other approach accounts for all the resources consumed along the life cycle of your product, from resource extraction from the natural environment, to their use in your main production process. These two approaches can give completely different results. A real study in the chemistry sector showed that when comparing the resource efficiency of two techniques to separate chemicals, the conclusion on which one is the most resource efficient varies depending on the level of evaluation chosen. When calculating the resource efficiency of the techniques at the level of the process and the plant, technique A has a lower resource efficiency than technique B. On the contrary, when calculating the resource efficiency at the life cycle level, the resource efficiency of technique A shows a higher resource efficiency than technique B. This illustrates that the level you will choose to calculate the resource efficiency of your product will have an impact on your results. Another choice that you will make and which will have a major impact on your results is the choice of the method that you choose to calculate the denominator of the resource efficiency ratio, that is to say the amounts of resources consumed.