Cancer/testis antigens (CTAs) are a kind of antigens that their expression mostly is restricted in testis and female’s genital organs. Tumor cells often express antigens whose expression is normally limited to germ cells. CTAs are composed of a vast gene family of closely related members and are commonly classified into two groups: the CT-X antigens that are encoded by the X chromosome and the non-X CTAs that are encoded by the autosomes. CTA are extensively and variably dispersed between tumors of diverse histotypes. CTA are broadly expressed in tumors, but not in normal tissue except for testis that is not available to the immune system, actually, the blood-testis barrier and the lack of HLA class I expression on the surface of germ cells avoid the immune system from the interaction with CTA proteins to be identified as non-self-structures. Consequently, CTA can be regarded as fundamentally tumor-specific targets. With extensive investigations on the function of this important biological molecules, their functions are somewhat revealed. Because of their high immunogenicity, tumor-limited, and biased expression, detection of these molecules provides unprecedented chances for further research and clinical development in the field of immunotherapy and cancer diagnosis. Also, growing evidence discloses that a number of CTAs stimulate epithelial mesenchymal transition (EMT) and generation of cancer stem-like cells, increasing metastasis, invasion and tumorigenesis. According to recent clinical attention, more features of CTA regulation are explored. CTA expression has been confirmed in a variety of human cancer tissues and some of them have been discovered to cause humoral and/or cellular immune responses in cancer patients, likewise, they displayed intertumor and intratumor heterogeneity in expression levels. CTAs are excellent targets for targeted tumor therapy, anticancer drug discovery, and diagnostic biomarkers, similarly, appreciated genes in the study of promoting tumorigenesis, immunotherapy, and malignant progression. This review summaries and classifies our current understanding of the complex and biased process of CTAs mRNA and protein expression in cancer, and provide the most current information on their function and regulation.