In this chapter, we described various aspects related to the role of molecules involved in redox balance and their involvement in the development of neoplasms. In normal cells, oxidative stress is typically maintained at moderate levels to balance reactive oxygen species within a functional dynamic range; however, tumor cells develop a highly structured system that allows them to sustain survival, proliferation, invasion, and chemoresistance by altering the fate of cell proliferation. Thus, when reactive oxygen species reach high concentrations, they can exert an antioncogenic effect by inducing apoptosis and inhibiting cell proliferation. In this sense, free radicals produced during tumor development play a dual role in cancer, which can either promote tumor progression or, under certain conditions, become a potential tool for the elimination of malignant cells.