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.

Eating disorders are commonly regarded as complex psychiatric conditions, although this perception might be a product of the high levels of physical and psychological co-morbidity that are present in many such cases, rather than being about complexity in the eating disorder per se. This paper will consider the reasons that eating disorders are seen as complex, and whether or not that perceived complexity should be seen as a genuine reason to deviate from existing evidence-based cognitive behavioural therapy (CBT) protocols. Case examples will be used to illustrate how complex presentations can require clinicians to work skilfully with relatively simple formulations to achieve the best outcome, rather than using unnecessarily complex formulations and treatments. The importance of clear supervision is also stressed, as it can play a role in clinicians’ perception of the need for a complex or simple formulation, it can support the clinician in developing a collaborative, focused and efficient formulation, and it can keep the clinician on track with an evidence-based CBT approach.

Obsessive-compulsive disorder (OCD) and related disorders such as body dysmorphic disorder (BDD) can take many different forms of presentations. The term ‘complex’ is common and inconsistently used in both OCD and BDD. Practitioners often refer to complex OCD or BDD when patients present with severe co-morbid problems, often in the context of personality difficulties, dissociation, difficult early relationships and trauma; or when the illness is chronic and debilitating with previous multiple treatment failures. Current best-evidence treatment protocols for both disorders focus heavily on exposure and response prevention (E/RP) but with moderate success, particularly in patients who are deemed ‘complex’, and often those with relevant shame and/or disgust-based past experiences. The aim of the present paper is to (a) describe factors that contribute to complexity in OCD and BDD, and (b) link these with theory and practice. We emphasize the importance of understanding both the function of OCD and BDD-related behaviours (rather than the content of obsessions or compulsion), and the context in which they occur such as the family. We illustrate complexity in OCD and BDD using real case material, using a functional and contextual approach to formulate the client's difficulties, and demonstrate how E/RP can be enhanced successfully with imagery rescripting, family work, and compassion-focused therapy.

Persistent complex bereavement disorder (PCBD) has been included in the appendix of the fifth edition of the Diagnostic and Statistical Manual for Mental Disorders as a condition for further study, and a new diagnostic category of prolonged grief disorder (PGD) is likely to be added to the International Statistical Classification of Diseases and Related Health Problems (ICD-11) (Maercker et al., 2013). Whilst there is increasing evidence that prolonged grief has distinct characteristics (Bryant, 2012), there are clinical features that overlap with post-traumatic stress disorder (PTSD), such as intrusive memories, emotional numbing, and avoidance of trauma or loss reminders. Here we describe how the cognitive model for persistent PTSD (Ehlers and Clark, 2000) and trauma-focused cognitive therapy for PTSD (Ehlers et al., 2005) have been helpful in treating persistent complex grief.

Cognitive behavioural therapy (CBT) for major depression is an effective treatment, but outcomes for complex cases, with co-occurring biological, psychological and social factors, are variable. Complexity factors can cause treatment to become diffuse, disorganized and over-complicated. At Step 3, disorder-specific protocols should be provided with therapy kept as simple as possible and delivered responsively, e.g. barriers to treatment should be tackled, ensure the client is well-prepared and seek to form a strong therapeutic alliance. At Step 4, if disorder-specific protocols have been ineffective, the priority is to formulate how complexity factors are interacting with the client's depression. An individualized formulation is used to carefully target these interactions. The treatment is still evidence-based and simple at the point of delivery, but there is greater emphasis on case-level interactions that are unique to each individual. Case examples are used to illustrate both approaches.

People with epilepsy are significantly more likely to have a mental health disorder than those without a chronic illness. The reasons for this are multiple but may include the mental health difficulties being perceived as complex due to the presence of a chronic illness. In part due to the apparent complexity of the co-occurring physical and mental illness, many are not offered evidence-based treatment (EBT) for the mental health disorder. There is little guidance to inform clinicians about the interventions to use to treat mental health disorders in people with epilepsy. The present paper reports a case of treatment for depression using a standard EBT in a young person with epilepsy. The patient also had clinically significant symptoms of anxiety and an eating disorder and would be considered ‘complex’ according to standard criteria. The intervention, however, was relatively simple and was delivered as guided self-help via 10 weekly telephone calls of approximately 30 minutes duration, and two follow-up calls at one month and three months post-intervention. Self-report and parent-report questionnaire measures were completed before and after the intervention, and at both follow-up time points. A blind-rated online diagnostic interview measure was also completed before and after the intervention. The young person and her family also completed a qualitative interview of their experiences of the intervention. This simple intervention was effective in working towards the client's goals, although pre–post measurement on standard measures was variable. This interesting case raises questions about whether patients with mental and physical comorbidities are complex, or just perceived as complex.

Cognitive behavioural therapy (CBT) is widely acknowledged as a treatment of choice for a range of psychological disorders, including those that affect criminal offenders and mentally disordered offenders. Meta-analyses of multiple studies provide evidence of CBT's efficacy and have led to its recommendation as a first line treatment for anti-social personality disorder (ASPD; NICE, 2009). However, there is comparatively little evidence about the effectiveness of CBT in relation to complex problems faced by offenders in prison and secure psychiatric services. The aim of this study was to examine how clinicians conceptualize and respond to complexity in the practice of CBT treatment with high-risk mentally disordered offenders. Using a qualitative methodology, semi-structured interviews were conducted with six practitioners. The interviews were analysed using thematic analysis. The first main theme was ‘critical challenges and issues’ which included subthemes addressing patient characteristics, the therapeutic context, ethical challenges, and treatment challenges. The second main theme was ‘overcoming obstacles’ which included subthemes on promoting safety, an emphasis on non-specifics, how to create an enabling space and creative practice in the application of CBT. The study emphasizes the importance of creating an enabling safe space by being ‘responsive’ to the patients’ needs and also emphasizes the importance of working to establish a baseline for ‘safety’ in all alliances wherever possible.