Experimental and theoretical studies of the interstitial and trapped-hole centers in alkali halides are surveyed. Emphasis is given on the centers of which the identification has been made relatively clearly. The most fundamental interstitial centers are the isolated atom interstitial center, the H center and the isolated ion interstitial center, the I center. The former has been identified clearly by EPR and ENDOR experiments. Some pieces of circumstantial evidence for the existence of the latter has been presented.