As a multi-layered motivation theory, Personal investment theory involves learners in the process of learning, due to multiple components, namely sense of self, facilitative conditions, and perceived goals of behavior. Investment has been described as an essential contributor to the second language learning process along with motivation. It specifies ‘how’ the actual learning occurs in a learning situation in various contexts (e.g., academic and non-academic) both formally and informally. Despite the scarce adoption of Personal investment theory in second language research, one can pose the question of how this theory can contribute to mainstream second language theorizing. This article aims to provide second language researchers with a detailed description of the Personal investment theory. The theory adopts a multi-layered approach to understanding the reasons for learners' investment in a certain domain. The paper presents a general picture of the key principles of Personal investment theory, illuminating the interactions between this theory and research in language education.