This dissertation is a modified systematic literature review with the purpose of exploring the following two questions. Firstly, what is Melanie Klein and Hanna Segal’s theory of symbolisation? This question is discussed in terms of Klein’s theory of symbolisation and symbol formation in the developing infant, and the contributions made by Segal to this theory. Secondly, how can this theory help understand the unconscious dynamics within the therapeutic relationship when working with a client diagnosed with Borderline Personality Disorder. Descriptive clinical vignettes are used to explore how one client’s developmentally arrested capacity for symbol formation manifests therapeutically. Emphasis is given on the therapist’s role in facilitating symbol formation processes between the client and the therapist, and eventually within the client using a substitute object. Klein’s theory of infant development is critiqued against infant research revealing shortcomings in her theory of symbol formation. A link is made between the structure of Borderline clients object relations and the inability to effectively negotiate the transition between the two developmental positions Klein (1946, 1948) termed the paranoid-schizoid and depressive position. Finally, the relevance of this link to symbolisation is discussed concluding the significance of symbolisation for the Borderline client in therapy.