Mastication or chewing is the process by which food is crushed and ground by teeth. It is the first step of digestion and it increases the surface area of foods to allow more efficient break down by enzymes. During the mastication process, the food is positioned between the teeth for grinding by the cheek and tongue. As chewing continues, the food is made softer and warmer, and the enzymes in saliva begin to break down carbohydrates in the food. After chewing, the food (now called a bolus) is swallowed. It enters the esophagus and via peristalsis continues on to the stomach, where the next step of digestion occurs.

Premastication is sometimes performed by human parents for young infants who are unable to do so for themselves. The food is masticated in the mouth of the parent into a bolus and then transferred to the infant for consumption.

Cattle and some other animals, called ruminants, chew food more than once to extract more nutrients. After the first round of chewing, this food is called cud.