Traffic shaping has an enormous impact on the provision of QoS (quality of service) guarantees. In this paper, the fundamentals of network calculus, a set of recent developments which provide a deep insight into flow problems encountered in computer networks are summarized and refined. By using network calculus, a general framework of traffic shaping is developed which includes greedy lossless shapers, traffic clippers and shapers with finite buffer to obtain, the input-output characterization of three traffic shapers, and how a traffic shaper can be used at all network elements to improve the efficiency of QoS mechanisms.