During the past two years, a wide range of plant responses have been found to be triggered by hydrogen peroxide that is generated in a genetically controlled manner by NADPH oxidases. Several studies have revealed examples of how changes in the concentrations of reactive oxygen species (ROS) are perceived and transferred into signals that change the transcription of genes. Moreover, both the chemical identity of a given ROS and the intracellular site of its production seem to affect the specificity of its biological activity, further increasing the complexity of ROS signalling within plants.