This is a detailed and pedagogical exposition of the effective Lagrangian techniques and their applications to high-energy physics. It covers the main theoretical ideas and describes how to use them in different fields, such as chiral perturbation theory and the symmetry breaking sector of the standard model and even low-energy quantum gravity. The book is written in the language of modern quantum field theory. Some of the theoretical topics treated are: decoupling; the Goldstone theorem; the non-linear model; anomalies; the Wess-Zumino-Witten term; and the equivalence theorem.