The Internet of Things (IoT) represents a paradigmatic shift in computing where everyday physical objects are embedded with sensors, software, and connectivity capabilities to collect, exchange, and act upon data. This chapter provides a comprehensive examination of IoT fundamentals, protocols, security challenges, and diverse applications across multiple domains. We explore the technological foundations, architectural frameworks, and communication protocols that enable IoT systems, while addressing critical security and privacy concerns. Through detailed analysis of real-world applications spanning smart homes, industrial automation, healthcare, and smart cities, we demonstrate IoT’s transformative potential. The chapter concludes with emerging trends including AI-enabled edge computing, blockchain integration, and sustainability considerations that will shape the future of IoT.