The quintessence of malignant transformation is the genetic alteration of the tumor progenitor cell, i.e. somatic mutation. The genetic change appearing at chromosome and/or gene level results in the disturbance of the balance of cell proliferation and differentiation. In solid tumors, including renal tumors, the basic genetic mechanism proved to be the loss of function of a specific gene pair caused by loss of the particular chromosome or chromosomal region (monosomy, deletion) or by mutation of the gene. In the well studied Wilms' tumor-aniridia-syndrome the distal part of 11p13 region is deleted. The responsible gene is the WT-1 tumor suppressor gene, a Zn finger type transcription factor. In the majority of cases the mutation of this gene leads to the tumor formation without cytogenetically detectable deletion. For manifestation of the tumor the functional damage of both alleles is needed. In other histological types of renal tumors a great variation of chromosome losses and gains, as well as translocations can be identified. In Wilms tumor of embryonic origin, tumor suppressor genes located on the short arms of chromosomes 16 and 17 play a role in the pathogenesis. Besides, the significance of abnormal genomic imprinting of IGF2 and H19 genes located on 11p15 has also been confirmed. If a part of the embryonic cells do not regress, they may develop to papillary carcinoma together with the appearance of trisomies of chromosomes 7 and 17 and loss of Y. In the transformation process from papillary adenoma to carcinoma, duplication of several chromosomal regions is involved (3q+, +8, +12, +16, +20). The origin of renal carcinoma developing from normal nephron cells is associated with a deletion of 3p and 5q+, while during the progression of the disease further variable chromosome losses appear (6q-, 8p-, 14q-, -9). Tumor-specific cytogenetic and molecular genetic changes confirm the morphological classification of epithelial renal tumors pointing at the relation of the various entities or their independence. Based on cytogenetic alterations, a sequential predictive model of renal tumors can be developed. Individual types, together with joining and sequential appearance of aberrations are in line with the multistep mechanism of carcinogenesis. At the same time, the specific cytogenetic and molecular genetic changes confirm the diagnosis, provide further information about the histological type and progression of the disease. In hereditary cases, the members of the family at risk can be identified by recognizing the possibly associating clinical symptoms and/or by detecting the constitutional mutation of the gene using molecular genetic methods. Consequently, the genetic study of renal tumors plays important role not only in diagnosis and choosing adequate therapy but also in prevention of the disease.