This paper presents a general critical picture of our present understanding of the frictional process. In particular it emphasizes the three main elements involved, namely the true area of contact, the nature and strength of the inerfacial bonds formed at the regions of contact, and the way in which the material around the contacting regions is sheared and ruptured during sliding. Most of the paper deals with metals but reference is also made to ceramics, lamellar solids, polymers, and elastomers. The paper concludes with a discussion of areas where further progress is desirable.