This systematic treatment of one of the oldest topics of mathematical analysis--the solution of functional equations--features numerous detailed proofs. Thorough and elementary in its approach, it will benefit upper-level undergraduates as well as graduate students.The text is divided into two parts: equations for functions of a single variable and equations for functions of several variables. Starting with equations that can be solved by simple substitutions, the first part examines the solution of equations by determining the values of the unknown function on a dense set. It also surveys equations with several unknown functions and methods of reduction to differential and integral equations. The second part begins with simple equations and advances to composite equations, equations with several unknown functions of several variables, and reduction to partial differential equations. The text concludes with explorations of vector and matrix equations.