This is a textbook for senior undergraduate and research students and a working reference for professionals in this field. The book shows how rules of gas-phase unimolecular and recombinant reactions can be calculated from first principles, and how these modern methods can be used to interpret, fit and understand experimental data. Applications of the methods given in the text include modelling of aerochemical and combustion processes. The book covers the fundamentals and applications of transition state and RRKM theory, of collisional energy transfer, and of the master equation. Extensive worked examples covering all aspects of the field are included.