Reactive oxygen species have strong oxidation activities and play an important role in the cell life.In cells，they possess and exhibit two contradictive activities，that is，oxidization essential for the cell life and protection of the cells against invasive microorganisms，and reactive oxygen species induce apoptosis aging and oncogenesis due to DNA damage，protein degeneration，cell impairment which are closely associated with various diseases.This chapter mainly describes the concept of reactive oxygen species and their relationship with physiological function，systemic diseases including cancerization and explains their effects on the oral cancer treatment.In addition，this chapter discusses on the new research methods and research prospects of reactive oxygen species.