This book contains papers based on selected talks given at the Dynamical Systems Seminar which took place at the Euler International Mathematical Institute in St. Petersburg in autumn 1991. The main problem of dynamics as Henri Poincaré formulated it one century ago is the investigation of Hamiltonian equations and in particular the problem of stability of solutions, and it has not lost its importance up to now. The aim of this collection is to give a wide picture of essential parts of the recent developments in qualitative theory of Hamiltonian equations such as new contributions to Kolmogorov-Arnold-Moser-theory and the study of Arnold diffusion and cantori. Furthermore, new aspects on infinite dimensional dynamical systems are considered. The book is intended for all mathematicians and physicists interested in nonlinear dynamics and its applications.