The main purpose of the following chapter is to present the theoretical underpinnings of self-efficacy which aim is to enable the reader to recognize the affective processes that have an impact on school success. Firstly, the origins of self-efficacy; definitions; its sources with mastery and vicarious experiences, verbal persuasion, and affective states are examined. Subsequently, the theories covering human and social agency, control beliefs, the self-efficacy strength model, and self-regulation in self-efficacy are discussed. Moreover, cognitive, motivational, affective, and selection processes are explained which is followed by a discussion of the concept of self-regulation deriving from cognitive-social theory. Due to the fact that this monograph deals with teaching a foreign language, the topics of self-efficacy in a school context and performance relationship are also explored. Finally, selected empirical studies on self-efficacy in students with and without dyslexia is presented.