Photoinduced light emission from fluorophores is highly sensitive to the formation of electron donor–acceptor complexes, energy transfer, and aggregation/disaggregation phenomena that may occur in the presence of certain chemical entities. The immediate changes in the emission spectrum, the possibility of quantifying them in precise ways, and the commercial availability of portable fluorometers have prompted the proliferation in the recent literature of diverse proposals for user-friendly fluorescent sensors. As the optically active receptor, these sensors may include common organic fluorophores (e.g., fluorescein), conjugated polymers (e.g., polyfluorene), quantum dots (e.g., CdTe), and other fluorescent or fluorogenic materials. Regarding the substrate onto which the fluorophore is immobilized, the choice of cellulose paper is increasingly popular, mainly due to its flexibility, its high availability, and its capability to wick water without becoming dissolved in it. Some commonly targeted analytes are explosives in leachates, heavy metals in water bodies, and health indicators in urine, among others. Besides the basics of wave physics, this chapter does not require any background on photochemistry from the reader, since photoluminescent phenomena, quenching and turn-on mechanisms, and fluorophore-analyte interactions are described in a didactical way.