We loosely define traffic theory to be the application of mathematical modelling to evaluate the relationship between demand, capacity and performance in a communications network. Traffic theory was developed over the last century and has been used extensively in the design of traditional telecommunications networks. It is currently not widely used to engineer the Internet, however, or to design its QoS enabled extensions. Similarly, the traffic theory for multiservice wireless networks may be said to be still in its infancy. In the talk we will provide examples from both wireline and wireless networking to demonstrate the crucial importance of traffic theory in designing cost effective networks offering the necessary guarantees of reliability and performance. We will outline cases of successfully applied theory and point to directions where further research is urgently needed.