Maryanthe Malliaris of the University of Chicago delivers the third in a series of Hermann Weyl Lectures at the Institute for Advanced Study, tracing model theory back to Hilbert's remarks on the continuum and non-Euclidean geometry before building up to her own research frontier. She walks through the field's core objects, theories, models, types, and ultrapowers, then focuses on Keisler's 1960s question about the saturation of regular ultrapowers and how Shelah's stability theory grew out of attempts to answer it. The lecture's payoff is showing how this decades-old problem has recently reconnected with seemingly unrelated areas: irregular pairs in Szemeredi's regularity lemma from combinatorics, differential privacy in theoretical computer science, and related questions elsewhere. Delivered to a live and remote audience at Simonyi Hall, this is a research-level talk aimed at mathematicians who want to see how abstract logic keeps resurfacing in other disciplines.