This volume contains the papers presented at the Third Alexander von Humboldt Colloquium on Celestial Mechanics. The papers cover a large range of questions of current interest, from the behaviour of dust particles to the stability of the solar system as a whole. The motions of asteroids and their classification into families are also discussed. Specific topics addressed are KAM theory, chaotic motions, resonances, Lyapunov characteristic exponents, perturbation theory and numerical integration.