The review considers the role of reactive oxygen species in the life of a plant cell. At the same time, attention is paid to both the negative aspects of their effect on cellular components (lipid peroxidation, protein carbonylation, and DNA damage) and positive functions (participation in signaling, stress response, and metabolism). The main types of reactive oxygen species and the sites of their generation in the plant cell are considered. It is concluded that reactive oxygen species, which inevitably arise in any aerobic organisms, should be considered as the most important regulator of a large number of plant processes, such as growth, development, metabolism, senescence, and stress reactions. Moreover, if the role of reactive oxygen species in signaling and under stress has been investigated in sufficient detail, the direct metabolic role has been studied relatively poorly, with the exception of lignin polymerization and softening of the cell wall, which indicates the need for further research in this area.