Rationale, aims and objectives The concept of patient or case complexity is relevant – and widely used – at all levels and stages of mental health service provision, but there have been few methodologically robust attempts to define this term. This study aimed to establish a consensus on factors contributing to patient complexity in adult psychological services using Delphi Methodology. Method Applied psychologists in a single urban/suburban UK National Health Service setting took part in a three-round modified Delphi study. Twenty-eight respondents in round one gave qualitative data on factors they considered when assessing complexity, which was subject to thematic analysis. Twenty-five respondents in round two rated how central/peripheral each theme was to their judgement using Likert scales. In a third round, twenty respondents addressed discrepancies and possible utilities of the emerging framework. Results Thirteen factors contributing to patient/case complexity (Active Severe/Enduring Mental Health, Current Coping/Functioning, Engagement, Forensic History, Iatrogenic Factors, Interpersonal Functioning, Neuro-Cognitive Functioning, Physical Health, Problematic Substance Use, Risk, Severity/Chronicity of Presenting Problems, Systemic and Socio-Economic Factors and Trauma) were identified with a high degree of consensus. All were rated as central to complexity. Conclusions We conclude that applied psychologists do have a shared understanding of complexity and make recommendations for further research validating, developing and applying this empirically derived framework. Keywords: psychological, complexity, definition, operationalising, framework development, clinical judgement