The broad field of molecular collisions is one of considerable current interest, one in which there is a great deal of research activity, both experimental and theoretical. This is probably because elastic, inelastic, and reactive intermolecular collisions are of central importance in many of the fundamental processes of chemistry and physics.One small area of this field, namely atom-molecule collisions, is now beginning to be ''understood'' from first principles. Although the more general subject of the collisions of polyatomic molecules is of great importance and intrinsic interest, it is still too complex from the viewpoint of theoretical understanding. However, for atoms and simple molecules the essential theory is well developed, and computational methods are sufficiently advanced that calculations can now be favorably compared with experimental results.