Mitochondria are not only a source but also a target of reactive oxygen species (ROS). However, the molecular mechanisms by which ROS affect mitochondrial function are poorly defined. In this issue, Screaton and colleagues report that ROS modulator protein 1 (ROMO1) links ROS and mitochondrial morphology and ultrastructure by modulating cristae remodeling and mitochondrial fusion that depends on the guanosine triphosphatase Opa1. Their work indicates how the oxidative milieu triggers mitochondrial shape changes.