NTA is one of the very few techniques that are able to deal with the measurement of particle size distribution in the nano-size region. This Guide describes the NTA technique for direct visualization and measurement of Brownian motion, generally applicable in the particle size range from several nanometers until the onset of sedimentation in the sample. The NTA technique is usually applied to dilute suspensions of solid material in a liquid carrier. It is a first principles method (that is, calibration in the standard understanding of this word, is not involved). The measurement is hydrodynamically based and therefore provides size information in the suspending medium (typically water). Thus the hydrodynamic diameter will almost certainly differ from size diameters determined by other techniques and users of the NTA technique need to be aware of the distinction of the various descriptors of particle diameter before making comparisons between techniques (see 8.7). Notwithstanding the preceding sentence, the technique is routinely applied in industry and academia as both a research and development tool and as a QC method for the characterization of submicron systems.