In recent years, the world has been moving toward connected smart and integrated devices like the Internet of Things (IoT), where devices communicate with each other with sensing and actuating capabilities. The diverse nature and wide range of applications raise various security issues and make these networks vulnerable. Beginning with an overview of the distributed applications of IoT, this chapter then presents various security issues, which are divided into layers. Various types of security attacks and threats are discussed to elaborate on the security problems in IoT networks. This chapter also covers security challenges and their countermeasures. Moreover, the chapter lays out the existing security models and solutions. The last section concludes the chapter by detailing many of the future trends in IoT.