The development of transition metal complex-based luminescent chemosensors has recently aroused increasing interest for protein biomarker labelling and detection, especially for the real-time diagnosis and treatment of disease. This is owing to their unique photophysical properties, particularly their long-lived and environmentally sensitive emission, which can be easily controlled via the structural modification of ligands. In this overview, we highlight recent examples of protein biomarker detection using group 8-9 metal-based luminescent chemosensors, including the frequently employed ruthenium(II) and iridium(III) complexes. Various mechanisms and sensing modes are described and compared, and the outlook and future directions of this field are discussed as well.