Tumor markers refer to a class of substances that are biosynthesized, released by tumor cells, or are host-responsive to cancer types during tumorigenesis and proliferation. These substances may be circulating substances that can occur in cells, tissues or body fluids and can be detected qualitatively or quantitatively in blood or secretions using chemical, immunological and molecular biology techniques. The analysis of the properties of such substances is used as a marker to identify tumor cells. Characteristics that a tumor marker should have: it must be produced by malignant tumor cells and measurable in blood, tissue fluid, secretion or tumor tissue; it should not be present in normal tissues and benign diseases; the tumor marker of a particular tumor should be detected in the majority of patients with that tumor. The tumor marker should be detectable before there is clear clinical evidence of tumor; the amount of tumor marker should reflect the size of the tumor; it can help to estimate the effect of treatment and predict the recurrence and metastasis of tumor to some extent. Most of the known tumor markers exist not only in malignant tumors, but also in benign tumors, embryonic tissues, and even normal tissues. Therefore, these tumor markers are not specific products of malignant tumors, but they are significantly increased in patients with malignant tumors, so they are also called tumor-associated antigens. In addition to some serum enzymes that can be quantified by measuring the viability, most of the proteins or other tumor markers without enzyme viability need to be determined by immunological methods. The sensitivity or specificity of a single marker is often low and cannot meet the clinical requirements. Theoretically and practically, it is advocated to measure multiple markers at the same time to improve the sensitivity and specificity. 2. Tumor markers are not the only basis for tumor diagnosis, but need to be combined with other means such as clinical symptoms and imaging examination for comprehensive consideration. The diagnosis of tumor must be based on histopathology or cytopathology. 3. Due to the individual differences of patients and their specific clinical conditions, the analysis of tumor markers should be combined with clinical conditions and compared from multiple perspectives in order to reach an objective and realistic conclusion. 4.Some tumor markers can also be abnormally elevated under certain physiological conditions or in some benign diseases, which need to be distinguished.