This chapter introduces the book "Stability of Functional Equations in Several Variables" by Donald H. Hyers, George Isac, and Themistocles M. Rassias. The book is part of the "Progress in Nonlinear Differential Equations and Their Applications" series, Volume 34, edited by Haim Brezis. The editorial board includes prominent mathematicians from various institutions. The authors provide a comprehensive introduction to the stability of functional equations in several variables, a topic that originated with S. Ulam's fundamental problem and was first partially solved by D.H. Hyers in 1941. The book covers classical results and current research, including topics such as approximately additive and linear mappings, stability of quadratic functional equations, approximately multiplicative mappings, and stability of trigonometric and similar functions. It also discusses related problems like the stability of convex functional inequalities and minimum points. The preface acknowledges the contributions of several mathematicians and expresses gratitude to those who provided useful suggestions and support during the publication process. It also notes the passing of D.H. Hyers, dedicating the book to his memory.This chapter introduces the book "Stability of Functional Equations in Several Variables" by Donald H. Hyers, George Isac, and Themistocles M. Rassias. The book is part of the "Progress in Nonlinear Differential Equations and Their Applications" series, Volume 34, edited by Haim Brezis. The editorial board includes prominent mathematicians from various institutions. The authors provide a comprehensive introduction to the stability of functional equations in several variables, a topic that originated with S. Ulam's fundamental problem and was first partially solved by D.H. Hyers in 1941. The book covers classical results and current research, including topics such as approximately additive and linear mappings, stability of quadratic functional equations, approximately multiplicative mappings, and stability of trigonometric and similar functions. It also discusses related problems like the stability of convex functional inequalities and minimum points. The preface acknowledges the contributions of several mathematicians and expresses gratitude to those who provided useful suggestions and support during the publication process. It also notes the passing of D.H. Hyers, dedicating the book to his memory.