Genetic instability, chromosomal or / and microsatellite is one of the characteristics of tumor cells. In tumor cells, both solid tumors and hematologic malignancies chromosomal instability expressed by the accumulation of structural and numerical aberrations of chromosomes. Chromosomal aberrations in tumors may be changes in specific, critical for the transition process, or secondary, which are the expression and at the same time cause genetic instability of cancer cells. The paper presents views on the mechanism of formation, methods of diagnosis and the importance of numerical and structural aberrations in the genetic instability in cancer.