For many medical imaging techniques, radioactive sources are used as tracers. These tests are potentially harmful for the patient and personnel and require a lot of precautions. This project is aimed at developing an alternative and safer way of bio-imaging, based on luminescent nanoparticles. Lanthanide-doped inorganic nanoparticles which are both magnetic and luminescent will be developed. Using luminescent materials which can be excited in the near-infrared, where biological tissues are relatively transparent, these particles can be re-excited from outside the body. In order to maximize the imaging contrast, several approaches are proposed. The decay time of the emission of inorganic phosphors is much longer than that of tissue autofluorescence, allowing to eliminate background signals by time-gating. Next, lanthanide ions have sharp emission peaks, enabling to apply narrowband filters to further suppress autofluorescence. Finally, using the emission from lanthanides in the so-called second optical window for bio-imaging eliminates all overlap with the autofluorescence signal. Particles with suitable optical properties and size will be coated to optimize their biosafety profile for use as in vivo multimodal contrast agents allowing combined optical as well as MRI (magnetic resonance imaging) detection. The latter experiments will be conducted in close collaboration with the Infinity small animal imaging lab (UGent) and Prof. Dr. Wim Ceelen, surgical oncologist at UGent.