Chapter 13 is devoted to the master equation, following mostly the derivation in Reif/Zwanzig books. Based on the golden rule in quantum mechanics, the master equation is derived first in the microcanonical ensemble, and then in the canonical ensemble. A specific example of relaxation phenomena related to nuclear magnetic resonance (NMR) is treated in detail with emphasis on the use of the close to equilibrium condition in the solution. The chapter ends with a statistical mechanics’ example of the principle of minimum entropy production suggested by Klein and based on the NMR model studied earlier.