New technological developments have created engineering problems where nonlinear effects have to be taken into account for a successful controller design. Unfortunately, the existing theory for general nonlinear systems cannot successfully deal with them. This book, which summarises the experience of the authors on control engineering applications over the past ten years aims to overcome this drawback by: • Giving back to modelling the central role that it deserves, and focusing on the structural aspects that can be exploited for the controller design. • Formalising mathematically current engineering practice, which is usually developed from practical experience and considerations, as opposed to theoretical analysis. • Proposing new control laws that sometimes are viewed as "upgrades" to the existing schemes. • Experimental validation, that confronts the new schemes with other theoretical studies or with industrial controllers. The fundamental concept of passivity and the perspective of control as a suitable interconnection of the system with its environment are key ingredients in all the developments of the book, which is primarily aimed at control-orientated graduate students and researchers, but which will also have value for both practising engineers and those concerned with the more theoretical side of the subject. The Communications and Control Engineering series reflects the major technological advances which have a great impact in the fields of communication and control. It reports on the research in industrial and academic institutions around the world to exploit the new possibilities which are becoming available.