Self or ego dissolution (SED) is a recurring, yet vaguely defined phenomenon often associated with positive therapeutic outcomes within clinical research on classic psychedelic substances. The aim of this thesis is to achieve a deeper understanding and improve terminological clarity of SED in a psychedelic context, here defined as research settings in which moderate to high doses of psilocybin are administered. SED is investigated from two different psychological perspectives to demonstrate how different understandings of the self and ego can contribute differently to the understanding of SED. Firstly, Freud’s theory of the subject constitutes a psychoanalytical perspective, which focuses on intra- and intersubjective processes in the constitution of the subject. Secondly, Rose’s theory of the subject constitutes a postmodern perspective, which focuses on contextual processes in the constitution of the subject. A qualitative systematic review (SR) is conducted to apply these theories analytically to empirical data on SED. The SR consists of 10 studies containing qualitative descriptions of subjects’ experiences with SED in a psilocybin-assisted psychotherapeutic (PAP) context. A thematic analysis of the subjects’ experiences is presented, and synthesized into four meta-themes that describe recurring characteristics of SED. These are 1) a stronger feeling of essence, 2) a greater feeling of connection, 3) a different bodily experience, and 4) a challenging but eventually blissful or meaningful experience. On the basis of the psychoanalytic analysis, it is suggested that SED can be understood as related to changes in the boundaries of the meta-psychological I and regression to primary processes including the fantasy and the skin ego. Additionally, based on the postmodern analysis it is suggested that SED can be understood as influenced by an unusual psy-context. Critical reflections on how the thesis contributes to the understanding of SED are presented and the strengths and weaknesses of the chosen theories and empirical data are discussed, specifically regarding interpretation bias, generalizability, data extraction, validity, causality, and eclecticism. It is concluded that the thesis provides insights important to psychological understandings of SED, insights that have potential clinical relevance. However, it is also argued that the investigation highlights that SED is a phenomenon that is difficult to capture and therefore should be investigated further from alternative psychological perspectives.