The use of classical mechanics to simulate atomic and molecular motion has a broad range of applications in science and engineering. This is clearly illustrated by the contribution in this volume, which address problems of both fundamental and technological importance. The cluster, surface, liquid, and interface molecular systems considered here are complex, consisting of many atoms, and strategies different than those used for smaller systems are needed to represent their potential energies and to simulate their motions. The description of such strategies is one focus of this volume. The other is to highlight some of the important applications of such large-scale trajectory simulations to problems of technological relevance.