This book explains how intelligence can be introduced into digital telephone networks. Beginning by outlining the basic principles of Intelligent Networks (IN), it tracks the application of the IN principle of separation of the service logic for advanced telephony services from the underlying call control infrastructures. The book explains how large public networks are updated to incorporate IN principles for advanced telephony services. This is illustrated using many practical examples which are set against a background of following the ETSI standards within the overall ITU-T framework of recommendations. This book is targeted at postgraduate telecommunications students, or engineers requiring a concise and practical introduction to IN.