As therapists we frequently use and hear the term ‘complexity’ in relation to clients, situations and settings. Although we may assume there is a shared understanding of what is meant by complexity, is this true? Do we really know what we mean by describing someone, or something, as complex? If we define complexity as ‘consisting of many different and connected parts, not easy to analyse or understand’ (Oxford English Dictionary, 2017), then we are probably describing intersections and interactions between different elements that can influence each other. Interestingly, the origin of the term derives from the Latin past participle plexus, meaning braided or entwined, which captures neatly the sense of the term ‘complex’ as meaning literally braided together. The breadth of this definition therefore may help to account for the diversity of the ways in which the term complexity is used in clinical settings. Continuing with the idea of the plait or braid, it also gives a sense of the number of threads or strands that could be incorporated within such a system. Complexity can derive from any source, and can interact with any part, so it can derive from the patient, the therapist, the therapeutic relationship or the healthcare setting; and each of these may interact with one or more parts. So from any source, complexity can affect processes and outcomes of care.

Disentangling what is meant by complexity is important for clients, therapists and services. Complexity at any level can mean that delivering interventions is difficult, time-consuming and costly. From the clients themselves, factors that can influence treatments may include co-morbidity, suicidality, physical health complaints, personality disorders or cognitive ability (e.g. Ruscio and Holohan, Reference Ruscio and Holohan2006). Therapists can also be the source of complexity or the complicating factor. Therapist factors can include experience, ability, supervision availability and reflective capacity. Some therapists see complexity everywhere, whereas others can miss the significance of complicating factors. Can the term be used inappropriately to describe chronicity or co-morbidity, and what is the impact on other professionals, or the client themselves, when the term complex is used in a loose or unclear way? In terms of settings or services, a frequently cited complicating factor is the treatment delivery protocol. Services may limit the number of sessions available that therapists can offer, or the mode of delivery, for example stipulating that sessions must be conducted within the clinic, restricting opportunities for home visits and behavioural experiments conducted in the ‘real world’.

Currently, case complexity is a heterogeneous concept, and there is not yet an accepted or standard understanding, language or framework to use in clinical practice. There is a long-standing tradition of published work on complex, challenging and difficult-to-treat cases that have contributed greatly to the theory and practice of therapy. However, there are probably instances in which complexity is in fact referring to chronicity or co-morbidity that may or may not have a complicating effect. In this Special Issue we explore what is meant by complexity in cognitive behavioural therapy and, just as importantly, what complexity is not.

Going beyond developing a shared understanding of the term, where it truly exists, what are the implications of complex processes, and how do they affect outcomes of care? Complexity in relation to diagnosis can also be considered as a method of formulation, including trans-diagnostic approaches as an alternative way of conceptualizing complex processes. It is commonly held that complex cases require longer or different forms of treatment to achieve improvement (e.g. Tarrier, Reference Tarrier2006). Although this view is often heard anecdotally, it has been difficult to disentangle what is meant by complexity and how this is translated into outcomes, although recently in IAPT (Improving Access to Psychological Therapies) there has been interest in determining how to detect complex cases early and match to high-intensity interventions to improve outcomes (Delgadillo et al., Reference Delgadillo, Huey, Bennett and McMillan2017).

This Special Issue also seeks to address the concerning gap between evidence-based practice and clinical practice. There is a frequently held view that patients selected for randomized control trials are not representative of ‘unselected’ patients in community settings (e.g. Goldfried and Wolfe, Reference Goldfried and Wolfe1996; Lilienfeld et al., Reference Lilienfeld, Ritschel, Lynn, Cautin and Latzman2013) and therefore this may lead to therapists dismissing the findings and relying on other means – such as clinical intuition – to make decisions about treatment selection and delivery. Coming from the other perspective, there are clinical researchers who do not prioritize interacting or co-existing factors as part of their treatment trials, and this also needs to be explicitly addressed. However, in both instances, a clearer articulation of what is meant by complexity and how it can be addressed is, in our view, essential to help bridge the gap within the field.

The overall purpose of this Special Issue is therefore to explore and attempt to understand complexity in relation to cognitive behavioural practice. Specialist areas will be investigated, including eating disorders, obsessive-compulsive disorder, bereavement, depression, physical health and offending behaviour, to explore topic-specific issues within particular areas, whilst at the same time trying to establish emerging principles that cut across different areas of practice. We hope this issue will help to move the field towards a common understanding through the development of a shared language and framework, whilst acknowledging that there are specific issues in different fields. This issue of papers will be the first to deal with this topic in this way, and we hope and anticipate it will make a significant step in the establishment of a shared understanding. We hope that these articles will be of interest to clinicians in the particular fields of practice, but also beyond those limits, as complexity is fundamentally an issue that cuts across all specialisms and settings.