 Much of Prior’s work on Tense Logic was initially motivated by the problems concerning the relationship between time and modality raised by the Master Argument of Diodorus Chronus and its fatalistic conclusion. Prior held a genuine interest in themes like indeterminism and the open future. One of his major concerns here was to allow for human freedom. In order to capture the notion of historical necessity underlying Diodorus’ argument, he opted for a branching representation of the interrelation of time and modality, which was first suggested to him in a letter by Kripke in 1958 (see Ploug and Øhrstrøm 2012). The Diodorean conception of modality as quantification over linear time was abandoned and replaced by the picture of a tree, whose branches depict alternative possibilities for the future. Prior (1967, Chapter 7) considered two different versions of branching time temporal logic, which he associated with the historical views of Peirce and Ockham, respectively. An early formal treatment of these ideas is provided in Thomason (1970; 1984) and Burgess (1978). Many technical results and further developments have been proposed since then, and we mention the most important ones below. For Prior’s motivational views on indeterminism and human freedom, see Øhrstrøm and Hasle (1995, Chapters 2.6 and 3.2) and Øhrstrøm (2019). A detailed discussion of the philosophical aspects of the theory of branching time is contained in e.g. Belnap et al. (2001) and Correia and Iacona (2013).