Abstract: Cancer of unknown primary (CUP) is a metastatic disease with significant cell heterogeneity and a highly malignant phenotype. It is difficult to detect the primary affected sites of CUP using routine medical examinations. Gene expression profiling, tumor epigenetics, and liquid biopsy are gradually being employed to identify the primary sites of CUP. Organ-specific therapy, targeted therapy, and immunotherapy based on these assays may have clinical applicability. They are expected to improve diagnostic accuracy and specificity in patients with CUP and increase their life expectancy. This article reviews the progress and prospect of advanced diagnostic techniques and treatment options for CUP.