An overview of expander graphs: Large, highly-interconnected, yet sparse graphs -- especially explicitly deterministically constructible ones. Lecture 16a of "CS Theory Toolkit": a semester-long graduate course on math and CS fundamentals for research in theoretical computer science, taught at Carnegie Mellon University. Resources for this lecture: "Expander graphs and their applications" by Hoory, Linial, and Wigderson. Taught by Ryan O'Donnell ( ~odonnell) Course homepage on CMU's Diderot system: Thumbnail photo by Rebecca Kiger ( )