Spark plasma and flash sintering process characteristics together with their corresponding sintering and densification mechanisms and field effects were briefly reviewed. The enhanced and inhibited grain growth obtained using these field-assisted densification techniques were reported for different ceramic nanoparticle systems and related to their respective densification mechanisms. When the densification is aided by plastic deformation, the kinetics of grain growth depends on the particles' rotation/sliding rate and is controlled by lattice and pipe diffusion. When the densification is aided by spark, plasma, and the particles' surface softening, grain growth kinetics is controlled by viscous diffusion and interface reactions. Grain growth in both cases is hierarchical by grain rotation, grain cluster formation and sliding, as long as the plastic deformation proceeds or as long as plasma exists. Densification by diffusion in a solid state via defects leads to normal grain growth, which takes over at the final stage of sintering. Various field effects, as well as the effect of external pressure on the grain growth behaviour were also addressed.