Lasers and chemical change is the study of radiation and molecules in dis­ equilibrium. The distinguishing feature of such systems is the extreme de­ parture from thermal equilibrium: the radiation is usually confined to a narrow frequency range, is well coll imated, and is far brighter than black body radiation; the chemical composition and also the distribution of mole­ cules over their different energy states are often markedly displaced from that expected at equilibrium. Such systems can be used as a source of laser radiation and, reversedly, lasers can rapidly and selectively displace mole­ cular systems from equilibrium. The subsequent evolution of the initially prepared state can then be monitored - again using lasers. One purpose of this book is to introduce the concepts required to d- cuss systems of radiation and molecules in disequilibrium. These include the physics of (laser) radiation and of radiation-matter interaction and molecular structure and spectroscopy. Excellent textbooks of these topics are available and our survey (in Chap. 3) is only intended to accent the es­ sential points, with special reference to atomic and molecular radiation physics. Considerably more attention is given to the topic of disequilibrium in chemical systems (Chap. 2). In particular we consider both inter- and intra­ molecular dynamics with special reference to energy requirements and energy disposal in chemical reactions and to what goes on in between - intramole­ cular energy migration.