Tight control of the cell cycle in eukaryotic cells exists to control proliferation, differentiation or apoptosis. These processes model and shape tissue and organ relationships in multicellular organisms. Two biochemical processes, protein phosphorylation/dephosphorylation and ubiquitin-mediated degradation drive cell cycle control. A multitude of pathways control cyclin-dependent kinase activities as the major event for cell cycle progression. Differentiation and apoptosis have cell cycle withdraw in common, while cancer and degenerative processes both show altered control of the cell cycle.