This chapter elaborates on different security aspects to be taken into accounts during the development and the deployments of IoT architecture. To make the reader about the security of the IoT based system, this chapter begins by defining the contemporary security requirements that should be considered to realize a reliable and trustworthy IoT environment. Then, the discussion extends to differentiate different concepts of IoT security i.e., threat, vulnerability, countermeasure, attacks, risks; and also explain how the concepts relate to each other. Later, a systematic taxonomy is presented for classifying IoT attacks according to IoT assets, where each class of IoT is further classified into more subcategories. Finally, the discussion of each elaborate different categories of IoT attack indicating their main security targets and possible IoT countermeasures.To sum up, this chapter intends to provide a comprehensive overview regarding IoT security vulnerabilities, threats, countermeasures, risks along with practices of handling them all through the following sections: • Security Requirements in Internet of Things • IoT threats, Attacks, vulnerabilities, and risks • Today's IoT attacks and Countermeasures • IoT attack surfaces • Summary and Learnt Lessons.