I will give a first-principles introduction to p-adic numbers and p-adic AdS/CFT. p-adic numbers are an alternative to the real numbers, and they have a hierarchical structure which lends itself naturally to a holographic construction. The bulk dual to a p-adic boundary is the so-called Bruhat-Tits tree, an infinite regular graph. A bulk-boundary correspondence can be set up starting from a classical action in the bulk, and correlation functions exhibit properties surprisingly similar to the ones derived in standard AdS/CFT. A version of gravitational dynamics can be defined on the tree in terms of a graph-theoretic Ricci tensor. The result is a non-linear theory of massless edge length fluctuations dual to an operator with some features in common with the stress tensor.