This book can be presented in two different ways; introduce a specific methodology to build adaptive Web sites and, presenting the main concepts behind Web mining and then apply them to adaptive Web sites. The authors begin by introducing the Web and motivate the need for adaptive Web site. The second chapter introduces the main concepts behind a Web site: operations, and related data structure, user sessions, etc. Chapter three explains the Web mining process and tools to analyze Web data, mainly focused in machine learning. The following chapter covers Web personalization, an important topic if we want to tailor our site to specific groups. Finally, the chapter contains nine case studies in which all the concepts previously applied to present a framework to build adaptive Web sites.