Background: The use of the terms complex and complexity alongside dementia is reflected in a number of policy documents. However, despite this, there is no accepted definition and there is a lack of evidence to support where the concept of complexity in dementia originated and how it is perceived, understood and experienced by people with dementia, their carers and clinicians working in the field. This lack of shared understanding is a concern as complexity is constantly referred to as one of the main reasons for admission into mental health NHS dementia inpatient wards. Aims: Adopting an interpretive description methodology this thesis used mixed qualitative methods to explore the concept of complexity in dementia within the setting of mental health NHS dementia inpatient wards. Interpretive description is a methodological approach orientated toward clinical practice (Thorne et al., 1997). It allows phenomena that are identified from clinical practice to be explored and the findings of the research to be applied back to practice. The study was conducted over three phases; phase 1 was an online electronic survey of national dementia leaders; phase 2 comprised individual interviews and a focus group with dementia clinicians; and phase 3 involved case studies of four patients with dementia whom mental health NHS dementia inpatient staff identified as being complex. Findings: The findings highlighted that complexity is constructed through a number of interconnected and interrelated domains; it is also not a static state but one thatâs fluctuates in acuity. These findings have been developed into âThe 3Fs Model of Complexityâ. The 3Fs stand for Fundamental, Flexible and Fluctuating. The Fundamental domain consists of five components which are always present in complexity. The Flexible domain consists of 15 components and a person with dementia may experience any number of Flexible domain components at any time. Moreover, the components of complexity may interact with one another both within and across domains. The third F in the 3Fs Model of Complexity is Fluctuating and represents the fact that components in both the Fundamental and Flexible domains have the ability to fluctuate in their acuity and effect on the person. It is this fluctuation that reflects the dynamic, evolving and changing nature of complexity. Conclusion: The research was inductively constructed and adopted a psychosocial approach to exploring complexity in dementia, in keeping with an interpretive description methodological approach. The 3Fs Model of Complexity may facilitate a more holistic view of a person with dementia than when âsymptomsâ are viewed in isolation of each other, as often happens in practice and as observed in data collection. Going forward, and subject to further testing the 3Fs Model of Complexity could help guide the selection of tailored, personalised interventions.