Tumor metastasis, the main characteristic of malignance tumor, is the primary cause of death for most cancer patients. The initiation of tumor metastasis involves complex signaling pathway within tumor cell and microenvironment, mediating primary tumor metastasis, invasion, survival and arrest in the blood circulation, and progressive growth at the distant site. The most research on the mechanism of tumor metastais will help understand the metastasis process, and identify promising molecular targets for cancer clinical diagnosis and treatment.