Abstract: A cancer of unknown primary (CUP), defined as a histologically confirmed metastatic cancer for which the primary site could not be identified after detailed investigations, accounts for 3%-10% of all malignancies. The diagnostic work-up in CUP relies mainly on imaging and histopathology with specific immunohistochemistry, while approaches of gene expression profiling and epigenomics help precisely identify the primary site in the genomic era. Nevertheless, these approaches have not been widely used in clinical practice. Hence, essential and crucial steps are necessary to facilitate the use of such approaches in clinical practice. CUP is a heterogeneous group of cancers, with low efficiency under traditional empirical treatment. However, identification of favorable subsets (accounting for 15%-20%) and organ-specific treatments could improve patients' survival. Moreover, molecular detection and application of next-generation sequencing offer the prospect of individualized targeted therapy with novel agents in CUP. Recently, with the development of immunotherapy, we are looking forward to breakthroughs in targeted therapy and immunotherapy for CUP.