 In this section, we discuss the language, semantics, and axiomatization of the basic tense logic TL introduced by Prior (1957; 1967; 1968), the founding father of temporal logic. Prior’s motivation for inventing tense logic was largely philosophical, and his ideas were strongly inspired by the use of tense in natural language. The innovative feature of Prior’s approach was that he treated propositions as tensed rather than tenseless. Technically, this was achieved by the introduction of temporal operators into the language, which were given a modal-logic type of semantics. In view of the pivotal role that tense is playing in his framework, Prior himself referred to his account as Tense Logic, whereas nowadays the more general expression Temporal Logic is prevailing.