Introduction and objective: The growing emphasis on evidence-based practice in psychotherapy has increased the demand for high-quality empirical data supporting treatments for personality disorders. Long-term psychotherapy presents unique challenges in this area, especially in designing and implementing randomised controlled trials. At the conceptual level, these challenges arise from the multiplicity of mechanisms and the complex, time-extended trajectory of change. At the organisational level, such research requires elaborate procedures and substantial financial and human resources. A recent additional difficulty is selecting primary outcomes and appropriate assessment methods amid a field-wide shift from categorical to dimensional models of personality pathology. Materials and methods: This article outlines key considerations in designing and implementing randomised controlled trials evaluating long-term psychotherapy for personality disorders. Results: Four critical domains are discussed: (a) selecting a trial design that is appropriately aligned with the underlying research question, (b) the influence of the theoretical framework on conceptualisations of change and outcome selection, (c) the methodological and clinical implications of operationalising change, including the use of valid, reliable, and theoretically congruent assessment tools, and (d) the impact of procedural decisions on outcome validity – particularly concerning the temporal dynamics of change, early effects, and the divergent priorities of key stakeholders such as patients, clinicians, researchers, funders, and policymakers. Conclusions: The article emphasises the need for pragmatic adaptations that preserve scientific rigour while enhancing clinical and translational relevance. It aims to inform future efforts to develop theory-driven, methodologically sound trials of psychotherapy for personality disorders.

personality disorders, long-term psychotherapy, randomised controlled trials, outcome indicators, dimensional models of personality disorders