Selenium (Se) is one of the trace elements that play an important role in many biological processes in the living body. Selenium acts in the body mainly in its forms called selenoprotein. Selenoproteins play a role in various events such as oxidative stress, immunity, cancer, inflammation, and endoplasmic reticulum stress. In selenium deficiency, the expression of selenoproteins and thus their activity decrease. In this case, some reactions such as increased oxidative stress, weakened immunity, endoplasmic reticulum stress, and inflammation cannot be prevented. The main source of selenium is food, and a diet poor in selenium causes selenium and therefore selenoprotein deficiency. This chapter will present information about the synthesis of selenoproteins and their role, especially in inflammation and endoplasmic reticulum stress response.