Generalised Anxiety Disorder (GAD) is a highly prevalent, chronic and costly mental disorder in children, that is characterised by excessive and uncontrollable worry about numerous topics, that occurs more days than not for a period of at least six months (APA, 2013). Our knowledge and understanding of worry and GAD in children remains a neglected area of empirical enquiry. Given the dearth of research conducted with children in this area, much of what is currently known about childhood GAD is derived from empirical studies conducted with adults. Within the adult literature, research has demonstrated that cognitive factors such as intolerance of uncertainty (IU), positive and negative beliefs about worry (PBW and NBW), negative problem orientation (NPO) and cognitive avoidance (CA) are particularly important in the development and maintenance of pathological worry and GAD, and these variables therefore constitute the basis of treatment for adult GAD. Treatment programs for child GAD, unlike those for adults, are almost uniformly transdiagnostic in nature and do not specifically target the cognitive variables demonstrated to be involved in the aetiology and maintenance of the disorder. This thesis investigated the cognitive variables of IU, NBW, PBW, NPO, and CA with respect to worry and GAD in children through a series of four studies, which have all been submitted for publication. The first study examined the cognitive variables of IU, NBW, PBW, NPO, and CA in a community sample of 114 children, aged 8 to 12 years. It was found that all child cognitive variables were significantly and positively related to child worry, and that parent worry, IU and CA were significantly and positively related to child worry. It was also found that the relationship between parent IU, NPO and CA and child worry was mediated by child IU, NPO and CA respectively.