Active networking is a step towards enhancing the static and inflexible structures of current networks. This book begins with an introduction into the interested area and a description of the basic mechanisms behind active networking. Examination of existing active systems and associated technologies is provided. The core of the book presents the design and implementation of a novel active router architecture that enables flexible network programmability based on so-called "user components". This active router is designed to provide maximum flexibility for the development of future network functionality and services. Its service composition method enables flexible programmability through the transparent integration of user components into the router's data path.