Explore the concept of treewidth in this graduate-level lecture from Carnegie Mellon University's "CS Theory Toolkit" course. Delve into why Constraint Satisfaction Problems (CSPs) and similar computational challenges are typically easier to solve on trees and series-parallel graphs. Learn about graphs with treewidth 2 and their properties. Examine tree examples, series-parallel graphs, and work through exercises to reinforce understanding. Gain valuable insights into fundamental concepts in theoretical computer science, preparing for advanced research in the field.