Bethe ansatz is a fascinating device in the theory of quantum integrable models. It enables the exact determination of the energy spectrum of dynamical systems ranging from integrable spin chains of magnetism to integrable models in high energy physics. It is particularly valuable for computing critical exponents in solvable models of statistical mechanics and plays a significant role in bootstrapping correlation functions in integrable gauge and string theories. Originally introduced in 1931 by Hans Bethe for solving the Heisenberg model of magnetism, the Bethe ansatz has since been extensively generalized and expanded. It now appears in various forms — the algebraic, analytic, functional, and thermodynamic — and has evolved into a universal framework for understanding integrability in quantum many-body systems. This book offers a comprehensive introduction to the Bethe ansatz techniques. It covers the factorized scattering theory and coordinate and algebraic versions of the Bethe ansatz, including the case of nested structures. Advanced methods based on functional relations among commuting transfer matrices and separation of variables are also addressed. A wealth of detailed calculations is included to facilitate the reader's swift engagement with the original literature.