The concept of a diffusion barrier is discussed in the context of thin-film metallization systems. The conditions that an effective thin-film diffusion barrier should meet are enumerated. The dominant role of defects in determining the kinetic properties of a barrier layer is pointed out, and the consequent importance of the method of deposition and the parameters prevailing during deposition are stressed. Examples of the practically important cases of the stuffed barrier, the passive compound barrier, and the sacrificial barrier are given, with special emphasis on the latter as applied to Al contacting of silicide layers. Unconventional ways of forming multiple or novel diffusion barriers are briefly mentioned as well.