Causal set theory (CST) is a discrete approach to quantum gravity which postulates that at the most fundamental level, spacetime is a locally finite partially ordered set. The partial order on the set is a proto-causality relation while local finiteness is a local discreteness requirement. In the continuum approximation the former corresponds to the spacetime causality relation and the latter ensures that finite volume regions in a spacetime contain only a finite number of "fundamental"

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... e events. CST is deeply rooted in the Lorentzian character of spacetime, where a primary role is played by the causal structure poset. Despite the assumption of fundamental discreteness, however, CST does not violate local Lorentz invariance in the continuum approximation. The combination of discreteness and Lorentz invariance on the other hand leads to a characteristic non-locality which distinguishes CST from most other approaches to quantum gravity. In this review we give a broad, semi-pedagogical introduction to CST, highlighting key results as well as some of the key open questions. This review is intended both for the beginner student in quantum gravity as well as more seasoned researchers in the field.