The present invention relates to diagnostic imaging agents for in vivo imaging. The imaging agents comprise a synthetic caspase-3 substrate peptide labelled with an imaging moiety suitable for diagnostic imaging in vivo. The invention also provides radiopharmaceutical compositions comprising the imaging agents, together with kits for the preparation of the radiopharmaceuticals. Also described are non-radioactive precursors suitable for the preparation of the imaging agents. The imaging agents are useful for the diagnostic imaging and or therapy monitoring in vivo of various disease states where caspase-3 is involved.