This lecture covers the fundamental concepts of diffusion, including Fick's laws, demonstration of diffusion in one-dimensional systems, solutions of Fick's second law, diffusion coefficient and random walk, temperature dependence, self-diffusion, force applied to diffusing particles, Darken equation, interdiffusion, Boltzmann-Matano method, scaling laws, Heaviside step distribution, and the Kirkendall effect. The instructor explains the concepts with examples and demonstrations.