CONTENTS Chapter I. Introduction Chapter II. Stability in the neighbourhood of a periodic torus Chapter III. Stability for arbitrary initial conditions Chapter IV. Transpositions, applications, prospects §1. Additional variables and an application to celestial mechanics §2. Transposition to other contexts and degenerate cases §3. Systems with (infinitely) many degrees of freedom §4. Steepness, quasi-convexity, and closed orbits Chapter V. Robust tori; Arnol'd diffusion §1. Robust tori and "normalization" §2. Arnol'd diffusion Appendix 1. Some Diophantine approximation Appendix 2. Gevrey asymptotic expansions References