Functional equations form a modern branch of mathematics. This book provides an elementary yet comprehensive introduction to the field of functional equations and stabilities. Concentrating on functional equations that are real or complex, the authors present many fundamental techniques for solving these functional equations. Topics covered in the text include Cauchy equations, additive functions, functional equations for distance measures, and Pexider's functional equations. Each chapter points to various developments in abstract domains, such as semigroups, groups, or Banach spaces, and includes exercises for both self-study and classroom use.