The aim of this chapter is to evaluate what information is needed in order to quantify the flows of ENM to the environment by reviewing the current state of knowledge. The life cycle thinking forms the basis of the evaluation. The first step in release assessment is the knowledge about the production and use of ENM. Data on production are crucial for the assessment, because they determine the maximal amount that could potentially be released. The different life cycles of products containing the ENM are determining the release potential. The knowledge about the product distribution is therefore key to release estimation. The three important life cycle steps that need to be considered are production/manufacturing, the use phase, and the end of life (EoL) treatment. Release during production and manufacturing to the environment may occur because large amounts of pure material are handled. During the use and EoL phase, experimental data from real-world release studies are preferred; however, in most cases release has been estimated or guessed based on standard knowledge about product use and behavior. The mass flows discussed in this chapter provide the input data to derive environmental concentrations needed for environmental risk assessment of ENM. The mass flows to the environment will also be needed for environmental fate models that are based on mechanistic description of the reactions and the behavior of the released ENM in environmental compartments such as water or soils.