The biostimulative effects exerted by various low power lasers in the visible and near I.R. might be a consequence of reactive oxygen species (ROS) production. Although high concentration of ROS leads to cell death, low and controlled concentrations may play an important role in cell activation. It is assumed that the light which is absorbed by endogenous photosensitizers induces the formation of minute amounts of ROS which are beneficial for the cell. In the present review we summarize recent experimental data providing evidence for ROS involvement in photobiostimulation of skin and sperm cells.