This paper reviews some fundamental aspects of point defect migration and agglomeration in crystalline Si. Both in-situ and ex-situ measurements were used to reach this target. Room temperature (RT) diffusivities of 1.5×10−15 and for I and V, respectively, were obtained using in-situ leakage current measurements, performed during and just after ion implantation. To follow the defect evolution and clustering upon annealing, ex-situ optical and electrical measurements were used. Low temperature (300–500°C) annealing causes the formation of point-like defects, while higher temperatures (500–800°C) are necessary to have defect clustering. Finally, a well-defined dose in pure Si) temperature (650°C) and time thresholds exist for the transition from I-clusters to extended {3 1 1} defects. When the transition takes place, both the optical and electrical defect properties undergo a dramatic change, suggesting an abrupt structural transition in the evolution from I-cluster to {3 1 1} defects. Kinetic lattice Monte-Carlo simulations used to model the defect agglomeration and growth confirm these results.