Nanodiamonds show great promise as photostable and biocompatible fluorescent markers and sensors for biological applications. Fluorescent defects in diamond particles can be employed to quantify for example magnetic fields and temperature at the nanoscale. Their versatile and robust surface chemistry allows for application specific surface termination and bio-functionalization. However, the use of fluorescent nanodiamonds in biologial systems beyond proof-of-principle experiments remains very limited. This presentation will discuss recent progress in the development of diamond nanomaterials for biological sensing and imaging applications. Material processing and the particles' optical properties will be addressed as well as bioconjugation strategies, the colloidal stability in complex media and targeted imaging approaches in vitro and in vivo.