Current cancer treatment paradigms are based on features of the primary tumour and extent of disease. For a group of patients with advanced metastatic disease, this crucial information is missing. Carcinoma of unknown primary (CUP) is a metastatic malignancy in the absence of an identifiable primary tumour. This thesis describes the use of molecular genetic testing to explore whether actionable, oncogenic driver mutations are present in CUP that have potential to better inform treatment decisions. Biologically relevant mutations were detected in the majority of CUP tumours, of which half provided a potentially novel treatment option not generally considered in CUP.