This book describes a new model, Relation Based Access Control (RelBAC) to handle the dynamics with full features of a general sense access control system. It is organized as follows: Chapter 2 analyzes the new challenges of the Web 2.0 such as the great dynamics in subjects, objects and in permissions. Chapter 3 lists existing access control models as the state of the art. Chapter 4 describes the RelBAC model and logic. We show the reasoning power of RelBAC in chapter 5. In Chapter 6, the extendibility of RelBAC is studied. Chapters 7 and 8 show applications of two important techniques of Sem.