Countless studies have implicated reactive oxygen species (ROS) and mitochondrial dysfunction in the ageing process. During cellular senescence, the ultimate and irreversible loss of replicative capacity of somatic cells grown in culture, several studies have reported increased levels of ROS associated with mitochondrial dysfunction and metabolic inefficiency. Moreover, studies have revealed that interventions modulating intracellular ROS can impact on the replicative lifespan of cultured cells, suggesting that ROS play a central role in the process. In this chapter, we present several protocols used for detection of (intra- and extracellular) ROS in live cells.