Quantum dots (QDs) are immobilized on the surfaces of magnetic Fe3O4-composite nanospheres (MNSs, see figure). The QDs exhibit intense visible-light emission in fluorescence spectroscopy and successfully facilitate, for the first time, in vivo soft-tissue imaging in live mice. The Fe3O4 nanoparticles respond to an external magnetic field by increasing the temperature of the surrounding environment (i.e., hyperthermia), which can be used therapeutically.