This book provides a comprehensive exploration of Nicotinamide N-Methyltransferase (NNMT), an enzyme with far-reaching implications across various fields of biomedicine. It delves into the multifaceted roles of NNMT in metabolic liver disease, unraveling its impact on hepatic steatosis, inflammation, and fibrosis through interconnected mechanisms of NAD+ depletion, methionine cycle disruption, epigenetic reprogramming, and gut-liver axis modulation. The book also examines NNMT's significance in systemic fibrosis across the heart, liver, kidneys, and lungs, and its potential as a therapeutic target for chronic organ failure. Furthermore, it discusses NNMT's critical role in hepatocellular carcinoma pathogenesis, from metabolic networks to therapeutic targets, including its functions in driving cancer cell survival, proliferation, invasion, metastasis, and drug resistance. With contributions from leading experts, this volume compiles cutting-edge research, offering valuable insights for researchers and clinicians. It highlights NNMT's potential as a diagnostic biomarker, prognostic indicator, and therapeutic target, and evaluates emerging interventions ranging from antisense oligonucleotides and small-molecule inhibitors to exercise-based non-pharmacological strategies, making it a vital resource for those engaged in NNMT research and its clinical applications. This work was supported by the National Natural Science Foundation of China (32260014).