In this chapter we will introduce the entropy S as a measure for missing information. It will turn out that the entropy S can strictly be defined in quantum mechanics by the average of the negative logarithm of the statistical operator \(\rho \) . When restricting to the diagonal elements of the statistical operator in some approximate basis, it can be shown that the entropy increases in time, i.e. \(S(t) \ge S(0)\) for \(t>0\) . Furthermore, the difference between micro-reversibility and macro-irreversibility will be discussed and the general postulates of statistical mechanics be formulated.