Carcinoma of unknown primary (CUP) is one of the 10 most frequent cancers worldwide. It constitutes 3-5% of all human malignancies. Patients with CUP present with metastases without an established primary site. CUP manifests as an heterogeneous group of mainly epithelial cancers recognised by distinct clinicopathological entities. The diagnostic work-up includes extensive histopathology investigations and modern imaging technology. Nevertheless, the primary tumour remains undetected most of the time. Molecular diagnosis with DNA microarrays demonstrates high sensitivity, but its prognostic contribution is still uncertain. Certain clinicopathological CUP entities are considered as favourable sub-sets responding to systemic platinum-based chemotherapy or managed by locoregional treatment. These sub-sets are: the poorly differentiated carcinomas involving the mediastinal-retroperitoneal nodes, peritoneal papillary serous adenocarcinomatosis in females, poorly differentiated neuroendocrine carcinomas, isolated axillary node adenocarcinomas in females or cervical nodal involvement by a squamous cell carcinoma. Patients who belong to the non-favourable sub-sets have a worse prognosis.