This chapter describes perturbed angular correlation (PAC) technique, which requires doping of the sample with suitable radioactive probe atoms, which emit a γ-γ cascade during their decay via an intermediate excited state. The interaction of the nucleus with magnetic or electric fields causes a Larmor precession of the nuclear spin and thereby a rotation of the intensity distribution. This rotation can be monitored by two coincidence counters set at fixed angle. The count rate exhibits a time modulation in accord with the precession frequency, superimposed on the regular exponential decay. The application of this time-differential perturbed angular correlation (TDPAC) to point defects is based on the fact that the local magnetic or electric field felt by the nucleus may be altered by the trapping of mobile defects, such as self-interstitial atoms and vacancies at the probe atom. Each defect complex created in this manner is characterized by a specific PAC time spectrum or by a characteristic frequency spectrum obtained from the Fourier transform.