The sintering of nanosized particles is a scientific and technological topic that affects the manufacture of bulk nanocrystalline materials and the understanding of the stability of nanoparticles. Owing to their extremely small size and the high surface to volume ratio, nanoparticles during sintering exhibit a number of distinctively unique phenomena compared to the sintering of coarse powders. Particularly, it is generally found that the sintering temperatures of nanosized particles are dramatically lower than that of their micrometre or submicrometre sized counterparts. Research has also shown that the grain growth during nanosintering consists of an initial dynamic grain growth stage that occurs during heating up and the normal grain growth stage that occurs mostly during isothermal holding. For nanoparticles, the effect of the initial grain growth cannot be ignored because that it is sufficient to cause the material to lose nanocrystalline characteristics. This review aims to bring to focus the understanding of the fundamental issues of nanosinteirng, including the thermodynamic driving force of nanosintering, non-linear diffusion and the kinetics of nanosintering, and the relationships between agglomeration, densification and grain growth. This review will also examine the effects of microstructure and processing variables.