The causal set theory (CST) approach to quantum gravity postulates that at the most fundamental level, spacetime is discrete, with the spacetime continuum replaced by locally finite posets or “causal sets”. The partial order on a causal set represents a proto-causality relation while local finiteness encodes an intrinsic discreteness. In the continuum approximation the former corresponds to the spacetime causality relation and the latter to a fundamental spacetime atomicity, so that finite volume regions in the continuum contain only a finite number of causal set elements. CST is deeply rooted in the Lorentzian character of spacetime, where a primary role is played by the causal structure poset. Importantly, the assumption of a fundamental discreteness in CST does not violate local Lorentz invariance in the continuum approximation. On the other hand, the combination of discreteness and Lorentz invariance gives rise 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.

量子引力的因果集理论（CST）方法假设，在最基本的层面上，时空是离散的，时空连续体被局部有限偏序集或“因果集”所取代。因果集上的偏序表示原始因果关系，而局部有限性编码内在的离散性。在连续统近似中，前者对应于时空因果关系，后者对应于基本时空原子性，因此连续统中的有限体积区域仅包含有限数量的因果集元素。 CST 深深植根于洛伦兹时空特征，其中因果结构偏序集发挥着主要作用。重要的是，CST 中基本离散性的假设并不违反连续统近似中的局部洛伦兹不变性。另一方面，离散性和洛伦兹不变性的结合产生了非定域性特征，这将 CST 与大多数其他量子引力方法区分开来。在这篇综述中，我们对 CST 进行了广泛的、半教学性的介绍，强调了关键结果以及一些关键的开放问题。这篇综述既适合量子引力领域的初学者，也适合该领域经验丰富的研究人员。