Material Flow Networks (MFN) are a modeling instrument in the field of material flow analysis (MFA) that helps to characterize current or future material and energy flows and stocks. Often, efficiency analyses such as life cycle assessments and cost accounting use such models to calculate the relationship between positive outcomes and negative impacts. The systematic integration of stocks makes it possible to analyze infrastructures that provide services, which is necessary in order to assess the effects of replacing material goods by services. The first part of this paper outlines Material Flow Networks as a period-oriented accounting system that systematically integrates stock and flow accounting. The second part is concerned with the question of how to construct the models and calculate data. From this perspective, the instrument becomes a modeling framework.