Since birth, our immune system is constantly bombarded with self-antigens and foreign pathogens. To stay healthy, complex immune strategies have evolved in our immune system to maintain self-tolerance and to defend against foreign pathogens. Effector T cells are the key players in steering the immune responses to execute immune functions. While effector T cells were initially identified to be immune promoting, recent studies unraveled negative regulatory functions of effector T cells in modulating adaptive as well as innate immunity. Owing to their critical and diverse functions in controlling immune responses, the functions and regulation of effector T cells have been a subject of intense investigation. In this review, key information about T cell immunity is summarized.