Network Calculus is a set of recent developments, which provide a deep insight into flow problems encountered in networking. It can be viewed as the system theory that applies to computer networks. Contrary to traditional system theory, it relies on max-plus and min-plus algebra. In this paper, we show how a simple but important fixed-point theorem (residuation theorem in min-plus algebra) can be successfully applied to a number of problems in networking, such as window flow control, multimedia smoothing, and bounds on loss rates.