Departing from the notion of a dynamical fuzzy set, we extend the concept of fuzzy logic, to introduce the dynamic fuzzy logic. The dynamic fuzzy logic is the base of the dynamic approximate reasoning, where the truth values and the inference rules are fuzzy, and change over time. The meaning of a dynamical fuzzy conditional proposition of the form IF A(t) THEN B(t), is clarified. We define also a dynamic linguistic variable as a variable whose truth value is represented as a word or sentence in a natural or artificial language whose meaning changes over time.