The mainframe has long been the backbone of the computing world, powering critical applications in various industries, including finance, healthcare, government, and transportation. Its unmatched reliability, scalability, and performance have made it a trusted solution for managing vast amounts of data and complex transactions. Despite the emergence of new technologies, the mainframe continues to hold a significant position in the IT landscape. However, with the increasing reliance on technology and the ever-evolving threat landscape, mainframe cybersecurity has become a critical concern for organizations. This book, "Mainframe Cybersecurity: A Comprehensive Guide," aims to provide a detailed understanding of mainframe security principles, best practices, and methodologies to protect these vital systems from potential threats and vulnerabilities. Our primary goal is to provide a valuable resource for IT professionals, security practitioners, mainframe administrators, and developers seeking to strengthen their knowledge of mainframe security. Whether you are new to the world of mainframes or an experienced professional, this book will serve as a comprehensive guide to help you navigate the complexities of securing mainframe environments. We hope that this book not only enhances your understanding of mainframe cybersecurity but also serves as a practical guide for implementing effective security measures to protect your organization's mainframe assets. As you progress through the chapters, we encourage you to apply the concepts and strategies discussed to your specific mainframe environment, keeping in mind the unique challenges and opportunities that it presents.