Explore the construction of the first explicit lossless vertex expanders in this Computer Science/Discrete Mathematics seminar lecture from the Institute for Advanced Study. Learn about graphs where every small subset of vertices has approximately as many neighbors as their sparsity permits, representing a significant advancement over previously known explicit vertex expanders like Ramanujan graphs, which only achieve half the optimal neighborhood size for small vertex sets. Discover how high dimensional expanders function as local-to-global amplifiers of lossless vertex expansion properties, providing crucial insights into this breakthrough construction. The presentation covers joint research with Jun-Ting Hsieh, Ting-Chun Lin, Alex Lubotzky, Sidhanth Mohanty, Ryan O'Donnell, and Assaf Reiner, delivered by Rachel Zhang from MIT on October 27, 2025.