The aim of this guide is to introduce the reader to nanoparticle metrology. The principles underlying metrology have been explained and their importance to nanoparticle size measurement has been highlighted, especially the role of traceability and uncertainty analysis in obtaining meaningful measurements. Several common techniques now routinely used in the analysis of nanomaterials are introduced. These are the high-resolution microscopic techniques of electron and scanning probe, which can resolve individual particles, and the ensemble methods of dynamic light scattering and small angle X-ray scattering which can analyse many thousands of particles simultaneously. It has been shown how these metrology concepts can be readily applied to these techniques. Finally the future direction of nanomaterial metrology is discussed.