The characterization of tumor-associated antigens and of human leukocyte antigen (HLA) class I molecule-binding peptide epitopes derived from these antigens has prompted the initiation of various vaccination trials aimed at inducing tumor-specific CD8+ T cells in persons with cancer. Sensitive and easy-to-perform T-cell assays that assess the frequency of tumor-reactive T cells are crucial for the evaluation and further development of vaccination approaches. This review focuses on a novel ELISPOT technique that allows quantification of tumor-specific T lymphocytes from peripheral blood by detecting antigen-induced cytokine secretion. Various ELISPOT methods using different antigen-presenting cells and different cytokines as read-out are described. T-cell analyses performed using the standard chromium release assay and the ELISPOT assay are also compared. Results from various clinical trials, including peptide and whole tumor cell vaccination and cytokine treatment, are now available and show the suitability of the ELISPOT assay for monitoring T-cell responses. To establish a basis for standardization and to further improve this technique, the first comparative quality assurance studies analyzing T-cell frequencies in different laboratories with the ELISPOT assay are being performed.