Generation of reactive oxygen species (ROS) occurs in seed not only during stress conditions, but also as a result of physiological processes. ROS are produced in seed in all developmental and germination phases like embryogenesis, including desiccation, seed ageing, imbibition, mobilization of storage reserves and germination manifested in radicle protrusion. Increase in ROS level during early embryogenesis and desiccation may lead to serious damage of embryo’s cells and disturb developmental processes. ROS generated during prolonged storage may promote the inability of seed to germinate. Reactive oxygen species may also protect seed and particulary the emerging seedling against attack by pathogens by oxidative burst which occurs when the expanding embryo ruptures the seed coat. In seed biology systems responsible for regulation of ROS production and scavenging play a key role.