The self-contained text is presented in an easy to understand fashion and all the necessary materials and information are included in order to appeal to a diverse audience with interests in difference and functional equations and functional analysis. Highlights of the text include discussions of the method of invariant means and the fixed point method,  the stability problems for the exponential functional equations, Hyers-Ulam stability, Hyers-Ulam-Rassias stability, Pexider equation, and superstability of the exponential function.