Section 16.1 contains the definitions and a discussion of the concepts of strictly stationary sequences and measure preserving transformations. It also presents Poincaré’s theorem on the number of visits to a given set by a stationary sequence. Section 16.2 discusses invariance, ergodicity, mixing and weak dependence. The Birkhoff–Khintchin ergodic theorem is stated and proved in Sect. 16.3.