Tracers that specifically target tumours are highly warranted for diagnosis and to monitor cancer chemotherapy response. However, as cancer cells arise from normal cells they do not substantially differ from the normal cells and therefore tumour specific targets are rare. Fortunately, the process of malignant transformation is associated with the up- or down-regulation of enzymes and transporters that play a crucial role in tumour growth. Consequently diagnostic imaging procedures have attained their major success with tracers that target enzymes and transporters that are over-expressed in tumours. The glucose transporters, the multi drug resistance transporters (MDRPs), several kinases and the family of cathepsins are prominent examples for enzymes and receptors that can be targeted for molecular imaging.