Semiclassical mechanics relates Newton's classical theory to modern quantum mechanics in a manner designed for the interpretation of the most recent experiments. The theory is physically intuitive and computationally accurate and is particularly powerful for large quantum numbers - hence it has special reference to atomic, nuclear, and molecular physics - but it also proves to be remarkably accurate even for the lowest quantum states.This book gives a comprehensive account of the application of the theory to molecular systems, with particular attention paid to the problems of non-separability. Although it is primarily a research monograph, the inclusion of problems at the end of each chapter makes this book equally applicable as a graduate text.