This book consists of contributions given at a symposium in honour of Leopold B. Felsen. They represent the state of the art in dealing with electromagnetic fields, their network theory representation, their computation and, finally, with system applications. The network formulation of field problems can improve the problem formulation and also contribute to the solution methodology. Network theory systematic approaches for circuit analysis are based on the separation of the circuit into the connection circuit and the circuit elements. Many applications in science and technology rely on computations of the electromagnetic field in either man-made or natural complex structures. Because different problems have their own combination of geometrical features, materials, scales and frequency ranges, no single method is best suited for handling all possible cases: instead, a combination of methods or hybridization is needed to attain the greatest flexibility and efficiency.