This chapter describes the foundations of Material Flow Networks for environmental management and gives an overview about their application fields. Material Flow Networks describe the flow of materials and energy within a defined system. The representation and evaluation of these material flows - especially when these flows have an impact on our environment and are caused by human business activities - has become one of the most important tasks of the so-called environmental management. The more familiar we become with the material and energy flows, the more we come to understand the relationship between human activities and our natural environment. The kind of techniques and tools required for material and energy flow analysis focuses on understanding the underlying material and energy transformations and the environmental impact of the resulting material and energy flows. Given the above, a possible definition of material and energy flow analysis is the process of collecting material and energy flow data and of computing derived values from the collection of data. The resulting material and energy flow model is a representation of the underlying system. The model must allow the user to evaluate different aspects of a system (see also, Schmidt, 1997): In input/output balances of companies, plants or production processes within the system refers to a site-specific view and a certain period of time, whereas in a life cycle assessment (LCA) a product or service is the item of interest, which usually is far beyond the temporal and spatial dimension of a common input/output balance. In fact, the same system is modelled in both cases, but interpreted with regard to different perspectives and boundaries.