The field of cytogenetics has had a great impact on many aspects of medical and basic sciences, including clinical genetics and, perhaps most notably, hematology and oncology. Tumor cytogenetics has for many years been dedicated almost exclusively to the study of hematological malignancies, mainly because these are more readily accessible. Important information has been obtained: cytogenetic changes in tumors are acquired, clonal abnormalities that are specifically associated with pathological subtypes of malignancies and premalignant conditions; they have a clear prognostic impact, and are used for choice of treatment, patient stratification, and assessment of minimal disease. Furthermore, molecular investigations of recurrent cytogenetic changes have led to the discovery of genes that play a determining role in leukemia and oncogenesis in general.