The book "Magnetic Reconnection" provides a comprehensive overview of the latest research findings in the main application areas of magnetic reconnection (MR), including planetary, solar, and magnetospheric physics, as well as astrophysics. It explains how research on magnetic reconnection, particularly concerning the Earth's magnetosphere, has significantly advanced through targeted observations from major satellite missions such as Cluster, Double Star, and Themis. The observations collected from these missions are complemented by numerous theoretical and model-based approaches that effectively utilize large computational resources. The book begins with an introductory discussion on fundamental questions regarding magnetic reconnection and then addresses topics such as collisionless magnetic reconnection, MHD structures in 3D reconnection, energy conversion processes, and fast reconnection mediated by plasmoids. Subsequent chapters focus on specific application areas such as magnetospheric day and tail reconnection, as well as comparing reconnection in planetary systems and reconnection in astrophysical systems.