"Preface These notes were prepared for lectures in plasma physics at the University of Tromsø and at the Department of Physics at the University of Oslo. The author would like to thank students and colleagues for their assistance. Liv Larssen from the Auroral Observatory in Tromsø helped, as so often before, in typing and editing parts of the manuscript and in particular preparing many of the figures. Her patience and help are gratefully acknowledged.Bjørn Lybekk was also very helpfulwith figures and solving a number of technical problems during the preparation of the text. The author is also indebted to Prof. Kristian Dysthe, the University of Bergen, Prof. Einar Mjølhus, the University of Tromsø, Prof. Lennart Stenflo, the University of Umeå and Prof. Mitsuo Kono, Chuo University for valuable discussion on the chapters discussing nonlinear waves in fluids and plasmas. Prof. Jan Trulsen, Institute for Theoretical Astrophysics, the University of Oslo, has offered help and given advice at all stages of the work. His friendly support has been indispensable. Many discussions with colleagues from Risø National Laboratory has been of great value. Also comments form numerous students over the years have helped to clarify many points in the text. A sabbatical year at the Norwegian Centre for Advanced Study (CAS), in the academic year 2004-2005, was most pleasurable and stimulating. I would like to express my appreciation to the staff at that institute. I am very grateful to Dr. Wojciech Miloch who patiently and carefully read large parts of the text and offered many useful comments and valuable criticism. Prof. Tünde Fülöp's initiative also contributed to the publication of this book, and I would like to thank her too"--