This thesis presents a literature review and two qualitative research papers that explore the under-researched and under-reported topic of iatrogenesis (unintended harm). There seems no clear theory within Counselling Psychology which encompasses the notion of iatrogenesis. Therefore, this research draws upon relevant theories from other domains. The research in this thesis is underpinned by Merton's (1936, 1968, 1972, 2016) sociological theory of unintended consequences, which supports a detailed exploration of what happens when two people meet in the social context of the consultation room. Each of the three studies which form this research will explore a different aspect of iatrogenesis. This is intended to support an exploration of unintended harm from various epistemological and methodological positions, and different analytical perspectives. For a conceptual consistency across the research, harm is defined as, "a negative effect [that] must be relatively lasting, which excludes from consideration transient effects ... [such as in-session anxiety or between session sadness, and] must be directly attributable to, or a function of, the character or quality of the therapeutic experience or intervention" (Strupp, Hadley, & Gomes-Schwartz, 1977, pp. 91-92). The theoretical grounding of the Literature Review is Merton's (1936) theory of the Unanticipated consequences of purposive social action, which I have used to explore the dilemmas involved when the unintended consequences of actions expected to engender helpful change, can result in an unexpected or unexpected outcome. The research begins with a review of the literature that reports the prevalence of iatrogenesis as 10% of the public attending therapy. Therapists in the role of client report the greatest level of harmful experiences, at up to 40%. In the review, the process of iatrogenesis is explored from the perspectives of quantitative, qualitative and mixed methods research. Each method reveals the strengths and weaknesses of the research approach when exploring the complex topic of iatrogenesis. The Literature Review concludes by suggesting there is a gap in the literature and indicates the relevance of qualitative studies as a means towards filling it. The second study will present an Interpretative Phenomenological Analysis (IPA: Smith, Flowers, & Larkin, 2009), of the experiences of psychotherapists in the role of client. Merton's (1972) distinction between 'insiders-outsiders' is applied, which in this study translates as 'insider' (client) and 'outsider' (therapist) roles, or positions. These positions help explicate potential mechanisms of change that are deemed to engender harmful experiences in psychotherapy sessions. A phenomenological approach was applied by interviewing counselling psychologists about their 'insider' experiences in their personal psychotherapy sessions. As their philosophical training is rooted in phenomenological, reflexive and humanistic training, counselling psychologists were assumed to be able to speak from the dual focus of being an informed client, as well as being an informed practitioner. Therefore, counselling psychologists were considered the most suitable group who would be best placed to help me explore the research question. Semi-structured interviews were conducted with four participants, all qualified psychotherapists. The data was analysed using IPA's methodology. The findings yielded three master themes: Competing world views: clashing epistemologies; How and by whom is therapy constructed?; and Making sense of an experience. The third study builds upon the Literature Review and broadens the findings of the IPA, by applying a qualitative method of Thematic Analysis (Braun & Clarke, 2006). The Thematic Analysis utilises Merton's (2016) distinction of the 'manifest' and 'latent' functions of purposive social actions. The notion of 'manifest' and 'latent' functions serves to explicate the experience of iatrogenesis from the perspective of psychotherapists delivering psychotherapy, who perceived their delivery of psychotherapy to have engendered unintended harm. The notion of functions serves also to explicate potential latent processes that can be obscured, and also the more subtle influences within and beyond the therapeutic space that impact upon what happens within the consultation room. The Thematic Analysis is framed by the increasing number of clients who are complaining to professional registration bodies regarding perceived harmful experiences during their psychotherapy. One response has been to introduce new codes of ethics. Applying Thematic Analysis, I conducted interviews with 20 practitioners from various modalities about their experiences of providing psychotherapy sessions. They reported their day-to-day experiences of 'do no harm'. The Thematic Analysis indicated three themes; 'Preparation for practice'; 'Boundaries'; and 'Issues of safety'. An overarching fourth theme was Professionalism. Transcending all the comments was the notion of tensions, which questioned: 'Is therapy an art or a science'? Implications are drawn for training, supervision, practice and the future. Across the three studies, I practice and research from a stance which is critical realist, which is to say we each edit the reality we perceive to accord it with our beliefs. My research position is that of a reflective scientist-practitioner, and I identity strongly with counselling psychology's philosophy and ethical value-base. The research stance is critical-realist.