Abstract: In every part of mathematics, there are ways of describing when two objects are "the same". When these objects are countable, or at least separable, the classes of objects under consideration (e.g., groups, graphs, metric spaces, etc) can often be realized as nice topological (or at least standard Borel) spaces, and the resulting notions of "the same" as Borel equivalence relations on those spaces. From here, the tools of descriptive set theory, as well as dynamics, group theory, and other areas, can be used to analyze the relative complexity of these equivalence relations, giving a global picture of classification problems across mathematics. In this talk, we will give a survey of the basic notions, examples, and results in this highly active area of set theoretic research. Speaker(s): Iian Smythe (UM)