Fluorescence resonance energy transfer (FRET) has been widely used as a spectroscopic technique in various areas such as structural elucidation of biological molecules and their interactions, in vitro assays, in vivo monitoring in cellular research, nucleic acid analysis, signal transduction, light harvesting, and metallic nanomaterials. Meanwhile, based on the mechanism of FRET, a series of FRET nanomaterials systems have been recently developed as novel chemical sensors and biosensors. Compared with those based on small molecules traditional FRET systems, the surface chemistry of nanomaterial has encouraged the development of multiple probes based on linked recognition molecules such as peptides, nucleic acids, or small-molecule ligands. This critical review highlights the design and the applications of sensitive and selective ratiometric nanoprobes based on FRET. We focus on the benefits and limitations of nano-FRET systems and their applications as chemical sensors and biosensors.