While bulk materials have constant physical properties independent of their size, the physical and chemical properties of a nanoparticle are dictated by its size. Accurate characterisation of mean size, size distribution and shape is key to the efficient manufacturing of high-quality nanomaterials. The EU-funded NanoPAT project plans to use three new real-time analytical tools that overcome problems and limitations of conventional characterisation technologies. The new process analytical technologies (PAT) will be photon density wave spectroscopy, optofluidic force induction and turbidity spectrometry. The innovative technologies will be combined with new data analysis methods to provide, for the first time, real-time analysis of particles on the nanometre scale with sub-minute temporal resolution. The present document shows a summary of the most relevant characterization techniques used to measure particle sizes and size distribution. These techniques are classified in three main approaches depending on the particle size distribution (PSD) analysis, which are ensemble, separation and counting methods. Each of the methods explained in the present document are based on different physical principles and present their own advantages and limitations. Only few of the techniques reviewed are valid for online/inline analysis.