 Reiter (2001) has proposed a (partial) solution within a framework, called the situation calculus, that has been very popular in KR especially in North America since it was proposed by John McCarthy, one of the founding fathers of AI (McCarthy 1963b, McCarthy 1986). The situation calculus is a dialect of first-order logic with some mild second-order features, especially designed to reason about actions. (One of its distinctive features is that of the so-called reification of semantic notions such as states or possible worlds (as well as truth predicates) into syntactic entities (‘situations’) in the object language.) For the sake of conformity in this entry and reasons of space, we will try rendering Reiter’s idea within (first-order) dynamic logic, or rather, a slight extension of it. (We need action variables to denote action expressions and equalities between action variables and actions (or rather action expressions) as well as (universal) quantification over action variables).