The study of chaos in quantum systems led to a thorough understanding of quantum billiards and to the statistics of the distribution of energy levels of those systems. Further, the recent technological developments in the production of microstructures present the possibility of manufacturing quantum systems in the ballistic regime.This volume represents the proceedings of the Miniworkshop on Nonlinearity: Chaos in Mesocopic Systems and the Adriatico Research Conference on Mesoscopic Systems and Chaos: A Novel Approach, both held at the ICTP, in Trieste, Italy, in the summer of 1993.These two events brought together physicists working in fields related to 'mesoscopic systems' and emphasized current advances and future possibilities. Vast areas were covered and although not everybody contributed to the book, those who have are able to show the many paths that this field opens, and also the importance of nonlinear phenomena in all branches of physics. We have organized the book to emphasize this importance, presenting a series of articles on models and different mesoscopic systems, followed by the microstructures where most of this research is concentrated.