 With these clarifications in place, we are now ready to look at what will be discussed in this entry. The most common strategy to obtain a concrete system of probability logic is to start with a classical (propositional/modal/etc.) system of logic and to ‘probabilify’ it in one way or another, by adding probabilistic features to it. There are various ways in which this probabilification can be implemented. One can study probabilistic semantics for classical languages (which do not have any explicit probabilistic operators), in which case the consequence relation itself gets a probabilistic flavor: deductive validity becomes ‘probability preservation’, rather than ‘truth preservation’. This direction will be discussed in   Section 2.   Alternatively, one can add various kinds of probabilistic operators to the syntax of the logic. In   Section 3  we will discuss some initial, rather basic examples of probabilistic operators. The full expressivity of modal probabilistic operators will be explored in   Section 4.  Finally, languages with first-order probabilistic operators will be discussed in   Section 5.