In the present paper an overview of metrology at the nanometric scale is given. After an introductory part on terminology and definitions concerning generic objects at the nanoscale, nanoparticles are taken as example to describe the overall features of these systems and typical characterization issues occurring from a metrological point of view. The case of nanoparticles is then further examined in depth with reference to gold nanoparticles, whose peculiar properties deriving from nano-scaled effects, like surface plasmon resonance, are discussed in detail, along with the state of the art on the main fabrication methods and applications in different sectors. Finally, a case study on the detection, by gold nanoparticles, of a contaminant used for food adulteration, namely melamine, is presented and analyzed.