This book is devoted to macroscopic models for traffic on a network, with possible applications to car traffic, telecommunications and supply-chains. The rapidly increasing number of circulating cars in modern cities renders the problem of traffic control of paramount importance, affecting productivity, pollution, life-style etc. The solution of the such problems has thus great socio-economical impact. Starting from classical and recent fluid-dynamic approaches to describe car traffic on a single road, the book develops an original theory to deal with arbitrarily complex networks. Moreover, efficient numerical schemes are obtained, real urban networks are well described and tests with real data are convenient and easy to implement.