How to construct dynamical systems obeying the first and second laws of thermodynamics: mean energy is conserved, and entropy increases with time? The book answers this question for classical probability (Part I) and quantum probability (Part II). A novel feature is the introduction of heat particles which supply thermal noise and represent the kinetic energy of molecules. When applied to chemical reactions, the theory leads to a nonlinear reaction - diffusion equations exhibiting oscillations or convergence to equilibrium.