The overall aim of this thesis was to better understand the potential clinical utility of psychological interventions for people with traumatic brain injury (TBI). To achieve this two questions are explored. Firstly, is there evidence for the clinical utility of early psychological therapies to improve outcomes following mild TBI. The evidence for this is reviewed in Chapter 2. Secondly, the factors associated with emotional distress in people with TBI are explored in Chapter 3, with particular interest in the psychological constructs associated with new third-wave therapies, Acceptance and Commitment Therapy (ACT) and Compassion Focused Therapy (CFT). Traumatic brain injury (TBI) is one of the leading causes of disability in young adults in the developed world; it is estimated that approximately 1.4 million people attending emergency departments having sustained a head injury (National Institute for Clinical Excellence, 2013). However, many have argued that prevalence figures are likely to underestimate the true prevalence as many people who sustain a TBI do not present to services (Kay, Newman, Cavallo, Ezrachi, & Resnick, 1992; Langlois et al., 2003; Mellick, Gerhart, & Whiteneck, 2003). Approximately 70-90% of TBIs are classified as being mild (Cassidy et al., 2004; Shukla & Devi, 2010). The majority of people who sustain a mild TBI are expected to make a full recovery with little or no intervention (Hall, Hall, & Chapman, 2005). However, there are a minority of people who continue to experience symptoms (including headache, nausea and cognitive complaints) for prolonged periods (Ruff et al., 1996). Whether these symptom complaints are due to impairments caused directly by the TBI or indirectly by psychological processes remains widely debated (Pertab, James, & Bigler, 2009; Rohling, Larrabee, & Millis, 2012). Regardless of the underlying cause, there remains a proportion of people who have sustained a mild TBI who continue to report symptoms that are considered disproportionate to their injury (Emanuelson, Andersson Holmkvist, Björklund, & Stålhammar, 2003). Symptoms which persisted beyond three months following mild TBI have historically been termed post-concussion syndrome or post-concussion disorder (Mittenberg & Strauman, 2000). The first aim of the present thesis was to examine the existing evidence for early psychological interventions to prevent the development of PPCS. Therefore, Chapter 2 systematically reviews current evidence from randomised controlled trials (RCT) exploring the evidence for whether early psychological interventions following mild TBI are effective in preventing the development of PPCS. People presenting with PPCS following a mild TBI often represent an area of unmet clinical need. At present, these people appear to 'slip through the net', with a lack of specialist expertise within conventional psychology services. As the mechanisms underlying PPCS remain unclear, people presenting with such difficulties may be referred to several different professions including, Clinical Psychologists, Neuropsychologists, Neurologists and Consultants in Rehabilitation Medicine. As such the review presented in Chapter 2, will be of interest to a wide range of professions. Furthermore, it is hoped that the findings of the review will aid the development of services for people presenting with persistent post-concussion symptoms. Secondly, the present thesis aims to explore the extent to which psychological processes predict emotional distress, after controlling for demographic and injury-related factors. More specifically, the study presented in Chapter two explores the contribution of self-criticism, self-reassurance and psychological flexibility to emotional distress in people with TBI. These processes are considered central to the models underpinning third wave therapies Compassion Focused Therapy (CFT; Gilbert, 2000) and Acceptance and Commitment Therapy (ACT; Hayes, 2004). Unlike traditional Cognitive Behavioural Therapy (CBT) approaches, CFT and ACT do not try to challenge the content of negative thoughts, but rather focuses on the relationship one has with these thoughts. CFT achieves this through development of the self-soothing system, encouraging individuals to be kind and nurturing to the self, as opposed to being self-critical. Similarly, psychological flexibility refers to how flexible one can be around negative thoughts and allowing the self to be present and non-judgemental to pursue behaviours in line with valued goals, even in the presence of potentially debilitating negative thoughts (Hayes, Luoma, Bond, Masuda, & Lillis, 2006). The primary reason for testing these constructs in relation to emotional distress was to assess the potential clinical utility of these third wave approaches. Exploring new therapeutic approaches is important for many reasons. Firstly, therapies are constantly evolving and exploring the utility of such approaches, not only provides more options for people needing such support but also allows for the refinement of existing approaches to ensure support is effective and appropriate. Secondly, evidence for traditional CBT for people with TBI, which remains the dominant approach, is limited with only two RCTs being reported in a recent review (Gertler, Tate, & Cameron, 2012). These studies yielded mixed results with one study (Fann et al., 2015) reporting results in favour of a CBT intervention compared with a control and one study (Simpson, Tate, Whiting, & Cotter, 2011) favouring the control group. CFT and ACT may offer advantages over traditional CBT in that they focus on process, rather than content. Interest in process-based CBTs is continuing to grow with the advantages of such approaches over more traditional approaches being demonstrated in many clinical populations (Hayes & Hofmann, 2017). Whilst in traditional CBT approaches, the content of negative thoughts is challenged and evidence-based alternatives are developed (Wells & Matthews, 1994), CFT and ACT focuses on the context and the relationship one has with these thoughts to promote clinical change. Therefore, in instances where the content of such thoughts is not irrational or invalid, challenging this would not be appropriate. It is likely that in people who have sustained a TBI, there will be occasions when negative thoughts would not be irrational with some people being unable to return to their old work due to physical or cognitive impairments.