The text presented here is an extended english version of a report by the authors which appeared in April 1983 at the Institute of Cosmical Research of the Academy of Sciences of the GDR in German. It covers several selected topics on nonlinear wave-plasma interactio,ll in a treatment based on a hydrodynamic plasma description. Thus, no attempt has been made to give a comprehensive view on all aspects of the interaction of strong electromagnetic waves with plasmas. The text is partly introductory and presents partly current results. The authors hope that it will be of interest to students and scientists not only in the field of plasma physics. The authors thank Akademie-Verlag, Berlin and Birkhiiuser-Verlag, Basel for their encouragement to prepare the English manuscript and Mrs. Ch. Geier for carefully typing the final off-set version. Klaus Baumgiirtel Konrad Sauer Berlin, in April 1986 Contents Preface 5 Introduction 9 General References 13 Part I Basic equations 15 1 Hydrodynamic plasma description 15 2 Basic equations for high-frequency processes 19 3 Basic equations for low-frequency processes 25 References 28 Part n Elements of linear wave propagation 31 4 Linear wave propagation in plasmas 31 4. 1 Linear wave equation 4. 2 Penetration of a plasma by an electromagnetic wave 34 4. 3 Resonance absorption 38 References 43 5 Structure resonances 45 5. 1 Resonances at s-polarization 46 56 5. 2 Sl'l'face wave resonances 5.