“Flow analysis methods evaluate the sustainability of a system (e.g. UM) by modeling one or more of its substance, material, or energy flows. The model describes, at the very least, the flows entering and leaving the system but inner circulation of materials can also be considered. They are valid for a specific period and stocks are usually modeled to respect the principle of mass/energy conservation. Flow analysis methods, in their standard forms, are quite simple and easy to implement. This simplicity is quite useful when complex systems like UMs are assessed. The simplicity allows for yearly assessment without requiring unmanageable amounts of data. Those methods are also the only ones that specifically offer information on stocks of materials and/or energy. This type of information can be quite relevant for decision makers even if it is not the main focus when environmental impacts or performances are assessed. The main drawback of using the flow analysis methods is the lack of clear environmental impacts description. For example, the UM studies do not show the environmental effects of those wastes if the flows of wastes are assessed with those methods. Flow analysis has been used to model and analyze different input data and this led to the creation of different methods.” (Beloin-Saint-Pierre et al. 2016)