Factors of measure-preserving systems are important to understand the structures behind ergodic theorems. We define point factors and Markov factors of a system \((X,\mu ,T)\) , discuss the relation between them and provide a characterization of (Markov) factors via subalgebras of \(L^\infty (X,\mu )\) and sublattices of \(L^1(X,\mu )\) . We present a few important examples of factors: the fixed factor, the rational spectrum factor, the Kronecker factor and the Abramov factor, with a further characterization of weak mixing and a discussion of topological models. We finally discuss conditional expectation operators and the disintegration of measures including the ergodic decomposition.