Internet of Things (IoT) refers to devices that are embedded with sensors, electronics, and software and can communicate with each other through the Internet. IoT has various advantages such as ease of information access, communication capability, and cost-effectiveness. These advantages facilitate IoT to be utilized in various applications such as smart cities, agriculture, industries, and healthcare. After the evolution of industry 4.0, IoT has been widely used to improve the performance of industrial systems. This chapter describes the fundamental concepts of IoT. It provides an overview of IoT architecture and the technologies that enable IoT applications. It also summarizes some programming languages and tools used for IoT development. It also describes the role of IoT in Industry 4.0 and the security issues involved in developing IoT applications. It also enumerates some of the applications of IoT.