A 'looked after child' (LAC) is a child or adolescent (under 18 years old) who is legally under the guardianship of a local authority. In 2014 there were estimated to be almost 70,000 LAC across the UK, with these numbers continuing to steadily increase each year. Of the 30,000 children who were new to looked after care in 2014, the majority were over the age of five years old and were placed in care services following significant abuse or neglect. Consequently, this population represents a sample of children who have experienced extreme, often prolonged, inter-personal trauma (referred to as complex trauma). Perhaps unsurprisingly, given the extensive trauma exposure, psychological outcomes for these children are some of the poorest of any group, with the majority meeting criteria for at least one psychiatric disorder. Broader outcomes are also poor: a minority successfully complete their schooling, they are at increased risk of exploitation/further abuse, and negative occupational and criminal justice outcomes are common in adulthood. At a societal level there is a substantial economic burden which further highlights the profound vulnerability of this group In sum, poor outcomes are the norm for LAC, presentations and needs are complex, and services to address these issues are extremely stretched. Given these considerations, an evidence-based framework is essential to ensuring that the support provided is appropriate, targeted and effective. Despite this, there is a surprising lack of robust research exploring how LAC's experiences of early complex trauma may impact on their outcomes and needs. While cognitive models of posttraumatic stress disorder (PTSD) provide a useful base for understanding the psychological impact of trauma, it remains to be established whether they can usefully be applied to children who have experienced complex trauma, often followed by continued instability and vulnerability. It also isn't clear whether trauma related cognitive processes also contribute to wider mental health outcomes that are common in this group (e.g., externalising behaviour). Understanding such processes is vital to developing targeted support for LAC, including ensuring the availability of appropriate psychological interventions, and crucially, the training of foster carers in how to support LAC. The wide ranging poor outcomes that are currently present in LAC highlight the urgent need to enhance current provision. The overarching aim of the proposed research is to explore how trauma-related psychological processes (such as thoughts about the trauma or memory of the trauma) may link LAC's experience of complex trauma with their psychological outcomes, as well as their trauma-related support needs. This work would make a major contribution to an under researched area by: (1) providing a large-scale exploration of the broad trauma-related mental-health profiles of LAC in the UK and, in particular, the trauma-related psychological mechanisms related to such profiles. Such information is essential to developing an empirical framework for improving outcomes for children who have experienced complex trauma; (2) providing important information on how LAC's trauma-related mental health profile is associated with their quality of life, education and placement success; (3) providing an in-depth exploration of what LAC view as their current support needs in relation to their experience of trauma, as well as identifying barriers to obtaining that support; and (4) exploring how their foster carers (i.e., their primary means of support) view the child's trauma-related support needs and barriers to these needs being provided. Taken together this research would provide the urgently-needed framework for understanding the needs of LAC and thus provide the foundation necessary for improving training and interventions in relation to this population.