Our book includes some state-of-the-art classical and non-classical approaches of data mining and given a well-balanced treatment of both theory and practice. This book is presented in a simple and lucid manner. Many case studies are provided for easy understanding of the readers. Hybrid approaches are new and included in this text. Application to intrusion detection attracts more researchers to this. Recently, Data Mining (DM) algorithms have been successfully applied in IDS in order to uncover new insights, associations and hidden patterns within large data set of logs and messages.