This article is a highlight of the paper by Jarvi et al. in this issue of Photochemistry and Photobiology as well as a brief overview of the state of the field of singlet-oxygen (1O2) detection in vivo. The in vivo detection of 1O2 using its characteristic 1270 nm phosphorescence is technically challenging. Nevertheless, substantial progress has been made in this area. Major advances have included the commercial development of photomultiplier tubes sensitive to 1270 nm light, techniques for spatially resolving the location of 1O2 at a subcellular level and more complex mathematical models for interpreting the kinetics of 1O2 emission from living cells. It is now recognized that oxygen consumption, photosensitizer bleaching, oxidation of biological molecules and diffusion of 1O2 can significantly change the kinetics of 1O2 emission from living cells.