Lyapunov exponents are asymptotic growth rates of compositions of linear maps. The main result of this chapter is an ergodic theorem that guarantees that these growth rates are well-defined for stationary sequences of (finite dimensional) linear operators. The plan for Chaps. 10 and 11 is as follows. In Sect. 10.1 we briefly motivate the Multiplicative Ergodic Theorem (MET). A precise statement of two versions of the MET is given in Sect. 10.2. In Sect. 10.3 we present an interpretation of Lyapunov exponents as volume growth, and in Sect. 10.4 we discuss applications and extensions of the results stated. Proofs are presented in Chap. 11 .