Abstract: Resonance energy transfer molecular imaging (RETI) can markedly improve signal intensity and tissue penetrating capacity of optical imaging, and have huge potential application in the deep-tissue optical imaging in vivo. Resonance energy transfer (RET) is an energy transition from the donor to an acceptor that is in close proximity, including non-radiative resonance energy transfer and radiative resonance energy transfer. RETI is an optical imaging technology that is based on RET. RETI mainly contains fluorescence resonance energy transfer imaging (FRETI), bioluminescence resonance energy transfer imaging (BRETI), chemiluminescence resonance energy transfer imaging (CRETI), and radiative resonance energy transfer imaging (RRETI). RETI is the hot field of molecular imaging research and has been widely used in the fields of biology and medicine. This review mainly focuses on RETI principle and application in biomedicine.