Cost accountants want to support decision making. To do so, they should not only calculate current costs and income, but they should also be able to tell managers how costs might change depending on managers' decisions. "How would costs change if we produced more or less products? How would they change if we produce more or less variants of products? How would they change if we increase the quality of our products?" Cost functions help to answer these questions and in this module, we make you familiar with them. First, we show you some typical cost functions and, second, we introduce a variety of methods that cost accountants can use to determine cost functions. Finally, we show you how accountants can document cost forecasts.