Adequate responses to environmental changes are crucial for plant growth and survival. However, the molecular and biochemical mechanisms involved are poorly understood and the signaling networks remain elusive. The accumulation of active oxygen species (AOS) is a central theme during plant responses to both biotic and abiotic stresses. In both situations, AOS can play two divergent roles: either exacerbating damage or activating multiple defense responses, thereby acting as signal molecules. Such a dual function was first described in pathogenesis, but also recently has been demonstrated during several abiotic stress responses. To allow for these different roles, cellular levels of AOS must be tightly controlled. This control can be attained through a diverse battery of oxidant scavengers. Perturbation of this scavenging capacity can lead to dramatic imbalances of AOS concentrations, leading to a modified redox status. Here, we summarize mainly the work done on plants that are deficient in catalase activity. These plants not only revealed the importance of catalase in coping with environmental stress but also provided us with a powerful tool to investigate the (multiple) roles of H2O2 in an intact plant system.