This volume contain thirty-one papers based on talks delivered at the 4th Workshop of the Nonlinear Control Network (http: //www.shef/?NCN4), held at The University of She?eld, 25 28 June, 2001. We would like to emphasize one peculiarity of our ?eld: nonlinear control is an example of a theory situated at a crossroad between mathematics and engineering science. Due to this position, nonlinear control has its roots in both ?elds and, as we deeply believe, can bring new ideas and new results for both domains. The book re?ects very well this double character of nonlinear control theory: the...