Cyclins represent a numerous group of proteins which modulate the cell cycle by binding to respective kinases (CDKs) and the formation of active complexes. Cyclin-dependent kinases are present in a cell throughout the cell cycle, activated by partner cyclins they phosphorylate numerous proteins, which are associated with the activation of transcription factors, subsequently involved in the replication of DNA and in modeling of the cell cycle. CDKs may be inhibited by two families of inhibitors: the INK4 family, which compete with cyclin D for binding with CDK4 and CDK6 and CIP/KIP family, which is capable of inhibiting CDK2 and CDK1. In contrast to healthy cells, tumor cells divide beyond any control due to various reasons: an overexpression of cyclins, inactivation of kinase inhibitors or due to the loss of integrity involving factors active in the control points. The cell cycle manifests a variable duration of 20 hours to 5-10 days. In most malignant tumors the cell population doubling time takes more than 50 days. Disturbances in cell cycle control and a disturbed secretion of cyclins, CDKs or CDKI are linked to the development of cancer in several locations.