Fluorescent sensors are an important part of the analytical scientist's toolbox. The use of fluorescent PET (Photoinduced Electron Transfer) sensors has seen particular growth in recent times. This Critical Review discusses recent growth areas in fluorescent PET sensors by emphasizing the modular features of the 'fluorophore-spacer-receptor' design. The occurrence of the dipicolylamine receptor in PET sensor designs is critically examined as a case in point.