Sintering of nanocrystalline materials has been investigated at two levels of observation. On the atomistic scale, molecular dynamics computer simulations have been used to study the initial stages of sintering when two or more nano-particles first come into contact. These simulations show that because of the high shear stresses that develop at the particle-particle contact areas, the particles undergo extensive densification in matters of picoseconds by dislocation motion. The simulations also reveal that the application of external stresses induces grain boundary sliding, and that sliding enhances the densification rate. Experimental measurements of densification on bulk specimens undergoing shear deformation provide information on the macroscopic behavior of the sintering of large assemblies of nanocrystals. The constitutive law for densification of nanocrystalline TiO 2 has been obtained. Attempts are made to relate the observed macroscopic sintering behavior of this system to the microscopic processes elucidated by the simulations.