Tumor markers refer to a class of biochemical substances produced by tumor tissues themselves that can reflect the presence and growth of tumors. They include embryonic antigens, glycan antigens, natural self-antigens, cytokeratins, tumor-associated enzymes, hormones, and certain oncogenes. Most of the tumor markers can exist in malignant tumors and some benign tumors, inflammation, and even normal tissues, there is no tumor marker with 100% specificity, so the elevation of tumor markers is not necessarily caused by tumors. For example, AFP may be elevated in patients with viral hepatitis and cirrhosis as well as in pregnant women, CA19-9 in patients with obstructive jaundice or rheumatism can be several times higher than the normal value, PSA can be mildly or moderately elevated in enlarged prostate, prostatitis, endometriosis can be mildly or moderately elevated in CA125, and even CEA can be mildly elevated in long-term smokers. Therefore, there are limitations in diagnosing tumors based on tumor marker tests alone. Mild elevation of tumor markers in a single examination or the results of each examination do not have great clinical significance, and only dynamic and continuous elevation has significance. Persistent elevation of one or several tumor markers found during physical examination should be highly valued and should be further combined with ultrasound, CT, MRI, endoscopy or PET/CT, and if necessary, pathology should be used to make a definitive diagnosis. Changes in tumor markers are of great significance in evaluating treatment effects and judging prognosis. If the tumor marker decreases from elevated to normal after surgical treatment, it indicates that the surgery is successful; if it does not exist or decreases slightly after surgery and then rises again, it suggests that there may be residual tumor after the surgery; if it decreases after surgery and then rises again after a period of time, it suggests that the tumor recurs or metastasizes. This kind of indication is often earlier than several months before the appearance of clinical symptoms. The rise and fall of tumor markers can indicate the prognosis of tumor patients, which is of guiding significance to the adjustment of treatment plan. A decrease in tumor markers after treatment indicates that the treatment is effective; if the tumor markers continue to rise after treatment, the treatment plan should be changed; if the tumor markers continue to rise after changing the treatment plan, it is often indicative of recurrence or metastasis. Another point that needs to draw the attention of clinicians: the concentration of tumor markers measured immediately after chemotherapy and radiotherapy may have a transient abnormal elevation, which is due to tumor necrosis, and the correct detection time is 6 weeks after treatment. The recommendation made by the Expert Committee on Tumor Markers of the Chinese Medical Association Branch of Laboratory Medicine is that, after the end of malignant tumor treatment, the tumor markers that were elevated before treatment should be monitored by regular follow-up according to the condition. Different tumor markers have different half-lives, so the time and period of monitoring are also different. Most experts at home and abroad suggest that the first measurement should be done 6 weeks after treatment; every 3 months within 3 years; every 6 months for 3~5 years; and every year for 5~7 years. If a significant elevation is found during the follow-up, it should be retested once after 1 month, and 2 consecutive elevations may predict recurrence or metastasis. This prediction is often earlier than clinical symptoms and signs, and helps timely clinical management.