Cell metabolism heavily relies on the redox reactions that inevitably generate reactive oxygen species (ROS). It is now well established that ROS fluctuations near basal levels coordinate numerous physiological processes in living organisms, thus exhibiting regulatory functions. Hydrogen peroxide, the most long-lived ROS, is a key contributor to ROS-dependent signal transduction in the cell. H O is known to impact various targets in the cell, therefore the question of how H O modulates physiological processes in a highly specific manner is central in redox biology. To resolve this question, novel genetic tools have recently been created for detecting H O and emulating its generation in living organisms with unmatched spatiotemporal resolution. Here, we review H O -sensitive genetically encoded fluorescent sensors and opto- and chemogenetic tools for controlled H O generation.