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Respiratory mitochondria undergo stochastic, intermittent bursts of superoxide production accompanied by transient depolarization of the mitochondrial membrane potential and reversible opening of the membrane permeability transition pore. These discrete events were named 'superoxide flashes' for the reactive oxygen species (ROS) signal involved, and 'mitochondrial flashes' (mitoflashes) for the entirety of the multifaceted and intertwined mitochondrial processes. In contrast to the flashless basal ROS production of 'homeostatic ROS' for redox regulation, bursting ROS production during mitoflashes may provide 'signalling ROS' at the organelle level, fulfilling distinctly different cell functions. Mounting evidence indicates that mitoflash frequency is richly regulated over a broad range, and represents a novel, universal, and 'digital' readout of mitochondrial functional status and of the mitochondrial stress response. An emerging view is that mitoflashes participate in vital processes including metabolism, cell differentiation, the stress response and ageing. These recent advances shed new light on the role of mitochondrial functional dynamics in health and disease. [ABSTRACT FROM AUTHOR]