This lecture discusses the Aharonov-Bohm effect, focusing on the impact of a solenoid inside an infinite cylinder on the paths of particles moving inside and outside the cylinder. It covers the relative phase, trajectories classified by topology, and the modified double-slit experiment. The lecture also explores the concept of magnetic monopoles and the behavior of particles on a ring with a solenoid. Exercises related to the Hamiltonian and eigenstates are included.