After gathering data on the raw material extractions and substance emissions associated with a product’s life cycle, the third phase of a life cycle assessment (LCA) is the life cycle impact assessment (LCIA). The inventory determines the quantities of materials and energy extracted, as well as the emissions to water, air, and soil. But, how do we interpret this inventory data? How do we link these values to their environmental impacts and compare the different impacts? The impact assessment phase addresses these questions. The different steps of the impact assessment are the classification of emissions into different impact categories, characterization of midpoint impacts, and damage (end point) characterization. The impact assessment methods are simple to apply, though their development can be relatively complex. This chapter presents each LCIA step, as well as a concrete example of application. Existing methods are then described in further detail. The developments needed to improve these methods are presented in Section 5.6.