Inorganic-organic hybrid nanoparticles (IOH-NPs) with a general composition [ZrO] 2 + [R Dye OPO 3 ] 2À , [Ln] 3 + n/3 [R Dye (SO 3 ) n ] nÀ , [Ln(OH)] 2 + n/2 [R Dye (SO 3 ) n ] nÀ , or [LnO] + n [R Dye (SO 3 ) n ] nÀ (Ln: lanthanide) are a novel class of nanomaterials for fluorescence detection and optical imaging. IOH-NPs are characterized by an extremely high load of the fluorescent dye (70-85 wt-%), high photochemical stability, straightforward aqueous synthesis, low material complexity, intense emission and high cell uptake at low toxicity. Besides fullcolor emission, IOH-NPs are suitable for multimodal imaging, singlet-oxygen generation as well as drug delivery and drug release. This focus review presents the material concept of the IOH-NPs as well as their synthesis and characterization. Their characteristic features are illustrated by selected in vitro and in vivo studies to initiate application in biology and medicine.[a] B.