ABSTRACT. We describe the basic theory of infinite time Turing machines and some recent developments, including the infinite time degree theory, infinite time complexity theory, and infinite time computable model theory. We focus particularly on the application of infinite time Turing machines to the analysis of the hierarchy of equivalence relations on the reals, in analogy with the theory arising from Borel reducibility. We define a notion of infinite time reducibility, which lifts much of the Borel theory into the class ∆ ∼ 1 2 in a satisfying way.Infinite time Turing machines fruitfully extend the operation of ordinary Turing machines into transfinite ordinal time and by doing so provide a robust theory of computability on the reals. In a mixture of methods and ideas from set theory, descriptive set theory and computability theory, the approach provides infinitary concepts of computability and decidability on the reals, which climb nontrivially into the descriptive set-theoretic hierarchy (at the level of ∆ 1 2 ) while retaining a strongly computational nature. With infinite time Turing machines, we have infinitary analogues of numerous classical concepts, including the infinite time Turing degrees, infinite time complexity theory, infinite time computable model theory, and now also the infinite time analogue of the theory of Borel equivalence relations under Borel reducibility.In this article, we shall give a brief review of the machines and their basic theory, and then explain in a bit more detail our recent application of infinite time computability to an analogue of Borel equivalence relation theory, a full account of which is given in [CH11]. The basic idea of this application is to replace the concept of Borel reducibility commonly used in that theory with forms of infinite time computable reducibility, and study the accompanying hierarchy of equivalence relations. This approach retains much of the Borel analysis and results, while also illuminating a part of the hierarchy of equivalence relations that seems beyond the reach of the Borel theory, including many highly canonical