Human tumor antigens recognized by T cells have been identified by means of various molecular biological and immunological methods, including cDNA expression cloning with patients' T cells and antibodies, cDNA subtraction using RDA and PCR differential display, systematic gene analysis such as DNA sequencing, CGH, DNA chip/microarray and SAGE, in vitro T cell induction and immunization of HLA transgenic mice. The identification of human tumor antigens has led to a better understanding of the nature of tumor antigens, anti-tumor immune responses in patients before and after immunotherapy, and tumor escape mechanisms. The information obtained from these researches has enabled us to develop and improve immunotherapy by attempting to overcome the identified problems, including intrinsically low immunogenicity of tumor antigens and several escape mechanisms, such as regulatory T cell induction. The existence of immunogenic unique antigens derived from genetic alterations in tumor cells, and the varied immunogenicity of shared tumor antigens among patients due to differing expression in tumor cells and immunoreactivity of patients, indicates that individualized immunotherapy should ideally be performed. The identified antigens will also be useful for development of diagnostic methods and molecular targeting therapy for cancer.