Damage formation during ion implantation in crystalline Si and its evolution as a function of annealing have been widely investigated both theoretically and experimentally in the last few decades. The increasing knowledge of the damage features helps scientists in the understanding and modeling of many phenomena such as transient enhanced diffusion of dopants and extended-defect evolution. Nevertheless, many questions on defect agglomeration and evolution upon annealing are still unsolved. Despite the large efforts devoted in recent years, it is not clear how point-like defects agglomerate, forming more stable and complex structures such as defect clusters, and how they evolve into extended defects. Such knowledge would be the basic foundation to implement simulation programs of device processing. In this chapter we review the main results on defect agglomeration, from the elementary defects …