Singlet oxygen (O) is the excited state of ground, triplet state, molecular oxygen (O). Photosensitized O has been extensively studied as one of the reactive oxygen species (ROS), responsible for damage of cellular components (protein, DNA, lipids). On the other hand, its generation has been exploited in organic synthesis, as well as in photodynamic therapy for the treatment of various forms of cancer. The aim of this review is to highlight the versatility of O discussing the main bioorganic applications reported over the past decades, which rely on its production. After a brief introduction on the photosensitized production of O, we will describe the main aspects involving the biologically relevant damage that can accompany an uncontrolled, aspecific generation of this ROS. We then discuss in more detail a series of biological applications featuring O generation, including protein and DNA labelling, cross-linking and biosensing. Finally, we will highlight the methodologies available to tailor O generation, in order to accomplish the proposed bioorganic transformations while avoiding, at the same time, collateral damage related to an untamed production of this reactive species.