The telecommunications landscape is on the cusp of a profound transformation, driven by the advent of Beyond 5G (B5G) and 6G networks. These next-generation systems promise to deliver ultra-low latency, massive device connectivity, and unprecedented reliability, enabling a new wave of applications that will redefine how we live, work, and interact. From smart cities that optimize energy and traffic flows in real time to autonomous vehicles navigating complex environments, from immersive metaverse experiences to intelligent industrial systems, B5G and 6G networks are set to unlock possibilities that were once confined to the realm of science fiction. At the heart of this revolution lies generative artificial intelligence (GenAI) and large language models (LLMs), which are not merely enhancing network capabilities but fundamentally reshaping how networks are designed, deployed, and managed. These AI technologies, powered by advancements in deep learning and transformer architectures, are enabling networks to become AI-native, with intelligence embedded at every layer, from edge devices to core infrastructure. This book offers a comprehensive exploration of the role of GenAI and LLMs in B5G networks, providing a blend of cutting-edge research, practical frameworks, and forward-looking insights for researchers, engineers, policymakers, and industry leaders.