This book highlights an in-depth exploration of gold nanoparticles (AuNPs), emphasizing sustainable, "green" synthesis methods that address both scientific and environmental challenges. This comprehensive guide is designed for researchers, students, and professionals in nanotechnology, detailing the synthesis, characterization, and versatile applications of AuNPs across fields such as biomedicine, environmental science, agriculture, and industry. Key topics include innovative synthesis approaches, from chemical and physical to plant- and microorganism-based techniques, along with the advantages and challenges associated with eco-friendly production. The book also discusses the functionalization and customization of AuNPs to optimize their properties for specific applications, such as targeted therapies, pollutant removal, and biosensing. By promoting interdisciplinary approaches and standardized green protocols, this book aims to inspire further advancements in sustainable nanotechnology and offer impactful solutions for real-world challenges.