Tooth wear is the result of four processes: abrasion (wear produced by interaction between teeth and other materials), attrition (wear through tooth-tooth contact), abfraction and erosion (dissolution of hard tissue by acidic substances). A further process (abfraction) might potentiate wear by abrasion and/or erosion. Knowledge of these tooth wear processes, and their interactions is reviewed. Both clinical and experimental observations show that individual wear mechanisms rarely act alone but interact with each other. The most important interaction is the potentiation of abrasion by erosive damage to the dental hard tissues. Saliva can modulate erosive/abrasive tooth wear, especially through formation of pellicle, but cannot prevent it.