A quantitative evaluation of the evolution of simple vacancy and interstitial point defects into more complex interstitial clusters and eventually into extended defects in Si is reviewed. Deep level transient spectroscopy, DLTS, provides a unique tool to monitor this defect evolution in ion-implanted Si, during exploration of a wide range of dose and temperature regimes. Anneal of simple point defects proceeds by recombination of vacancies and interstitials until, in the case of Si ion-implanted Si, only interstitial point defects remain. As the ion dose and the annealing temperature are increased, electrical signatures related to cluster defects appear. The concentrations of O and C, as well as the shallow doping levels effect the formation and dissolution kinetics of defect clusters. The number of available free interstitials controls the thermal stability of these defect agglomerates. At the defects evolve into extended defects, observable by TEM, the interstitial clusters exhibit greater thermal stability and exist simultaneously with {311} imperfections.