The coarsening or Ostwald ripening of particles occurs through interface motion controlled by diffusion or interface reaction. In contrast, grain growth results from grain boundary motion driven by grain boundary energy and curvature. Statistically, smaller grains or particles shrink and disappear, leading to an increase in the average grain or particle size. The grain boundary motion is affected by solutes, second-phase particles, and surface grooves. These drag effects, along with faceting and roughening transitions, can result in abnormal grain growth. Sintering involves both densification and grain growth, as the total interface energy decreases due to both processes. Grain boundary motion is influenced by pore-boundary interactions during sintering. The sintering trajectory, depicted as the relative density versus grain size plot, is flat in the intermediate stage where pore channels effectively anchor the grain boundaries. The trajectory becomes steep in the final stage because grain boundaries can move freely as the number of small, isolated pores decreases.