Modern psychology has long focused on the importance of the body as the basis of the self. However, this focus concerned the exteroceptive body, that is, the body as perceived from the outside, as when we recognize ourselves in the mirror. This influential approach has neglected another important dimension of the body, namely the interoceptive body, that is, the body as perceived from within, as for example when one feels her racing heart. INtheSELF, however, goes beyond this approach, aiming instead to show how interoception and interoceptive awareness serve the unity and stability of the self, analogous to the role of interoception in maintaining physiological homeostasis. INtheSELF developed novel, pioneering methods for the study of causal relationships between interoceptive and exteroceptive awareness, allowing us to test how these two sources of information about the self interact to reflect the balance between stability and adaptation; how their inter-relation is built in parallel to the development of self-awareness in early childhood and adolescence; and the role that their interaction has for social relatedness. INtheSELF elucidates how humans navigate the challenging balance between inside and out, in terms of both the individual’s natural (interoception vs. exteroception) and social (self vs. others) embodiment in the world. The questions we are asking are timely, and the answers that INtheSELF gives can be impactful within and beyond academia. In a society which constantly exposes us to external, idealised images of the body and the self, my project stands to offer a unique window of insight into how our self can be built from the inside out.