Somatostatin derivative compounds of general formula (I) that may be readily labelled with the isotope fluorine- 18 and that have affinity and selectivity for cellular somatostatin receptors are provided. The labelled compounds are useful clinically as radioactive tracers in various in vivo imaging applications (for example, using positron emission tomography (PET) and related techniques) to detect somatostatin-expressing cells and tissues, including tumours, or as therapeutic agents.