The application of biologically active peptides labelled with positron-emitting nuclides has emerged as a useful and interesting field in nuclear medicine. Small synthetic receptor-binding peptides are currently the preferred agents over proteins and antibodies for diagnostic imaging of various tumours. Due to the smaller size of peptides, both higher target-to-background ratios and rapid blood clearance can often be achieved with radiolabelled peptides. Hence, short-lived positron emission tomography (PET) isotopes are potential candidates for labelling peptides. Among a number of positron-emitting nuclides, fluorine-18 appears to be the best candidate for labelling bioactive peptides by virtue of its favourable physical and nuclear characteristics. The major disadvantage of labelling peptides with 18F is the laborious and time-consuming preparation of the 18F labelling agents. In recent years, various techniques have been developed which allow efficient labelling of peptides with 18F without affecting their receptor-binding properties. Moreover, the development of a variety of prosthetic groups has facilitated the efficient and site-specific labelling of peptides with 18F. The 18F-labelled peptides hold enormous clinical potential owing to their ability to quantitatively detect and characterise a wide variety of human diseases when using PET. Recently, a number of 18F-labelled bioactive peptides have shown great promise as diagnostic imaging agents. This review presents the recent developments in 18F-labelled biologically active peptides used in PET.