Abstract: Cancer of unknown primary (CUP) is a heterogeneous tumor type that has been diagnosed as a metastatic tumor by pathological examination, but the primary tumor cannot be identified through comprehensive clinical examination. The incidence of CUP accounts for approximately 1%–2% of all tumors. CUP progresses rapidly and has a short course. The treatment and prognosis of patients with CUP are closely linked to the primary site. In clinical settings, identifying the primary tumor remains challenging. Scholars have focused on improving the detection rate. Novel technologies, such as gene expression profiling, high-throughput sequencing, epigenetics, and liquid biopsy, have been successively applied to identify the primary tumor of CUP accurately, sensitively and specifically. With the guidance of molecular diagnosis, targeted therapy, immunotherapy, and combination therapy will usher in the era of precision treatment for CUP, which may become a typical example for individualized therapy.