Abstract: Dysregulated cell cycle-mediated cell transformation and uncontrolled cell growth are some of the fundamental biological features of malignant tumors. Thus, understanding the mechanism(s) involved in the cell cycle will facilitate the discovery of the nature of tumor initiation and progression as well as the molecular mechanism of tumor malignancy. Consequently, biomarkers and drug targets for early diagnosis and treatment would be obtained. This study was designed to analyze in detail the key regulators of cell cycle and their importance in carcinogenesis and clinical prevention, diagnosis and treatment, and anti-cancer drug discovery. The biological impact of the dysregulated cell cycle on tumor occurrence and malignancy development is systematically described. The theoretical and practical significance of the cell cycle regulation in tumor diagnosis and therapy are also provided.