This lecture covers the fundamentals of rate theory in chemical reactions, including classical and quantum transition-state theories, tunneling effects, and the use of ring-polymer molecular dynamics. It discusses the concepts of flux-side correlation functions, recrossing, and the Bennett-Chandler approach, emphasizing the importance of optimal dividing surfaces and asymmetric barriers. The lecture also explores the implications of symmetric and asymmetric barriers on reaction rates, as well as the crossover temperature in deep-tunneling regimes.