Nature has designed peptides to stimulate, inhibit or regulate many body functions. The development of radiolabelled peptide-based radiopharmaceuticals for imaging a variety of tumours, infection/inflammation and thrombus has seen a new era in nuclear medicine. Recently, a number of 99Tcm-labelled bioactive peptides have proven to be useful diagnostic imaging agents. Due to their small size, peptide molecules exhibit favourable pharmacokinetic characteristics, such as rapid uptake by target tissue and rapid blood clearance, which potentially allows images to be acquired earlier following the administration of a 99Tcm-labelled peptide radiopharmaceutical. The challenge is to label bioactive peptides with 99Tcm with high specific activity without impairing the biological properties of the peptides. Molecular engineering techniques now permit synthesis of a wide range of biologically active peptides that carry chelating groups in their structure without affecting their receptor binding properties, thus permitting a high specific activity product. This review presents recent developments in 99Tcm-labelled small peptides and their potential applications in the imaging of various types of diseases. In addition, the different techniques for radiolabelling small bioactive peptides, the pharmacokinetic properties of peptides, and their potential as diagnostic imaging agents are also addressed.