This review discusses the impact of molecular genetics on the diagnosis and prediction of the prognosis for renal-cell tumors. Through the use of molecular techniques, it has become possible to divide renal-cell tumors into genetically and biologically well-defined entities. The new classification has many advantages over morphological ones: (1) in contrary to tumor phenotype, genetic markers are constant during tumor progression and allow a precise diagnosis; (2) genetic analysis discriminates between papillary (multiple origin) and nonpapillary renal-cell carcinomas (solitary tumors); (3) genetic analysis allows a precise diagnosis of renal adenoma; (4) the new classification system offers a genetic prognostic system; and (5) the new diagnostic-prognostic system could easily be adapted to modern detection systems such as automated sequence and fragment-length analysis, which allow a quick and correct diagnosis with a high throughput.