Rennet is an extract from the fourth stomach of young ruminants, such as cows, goats, and sheep. This extract contains a number of enzymes which are designed to help these animals digest their mother's milk, and when added to milk, rennet will cause the milk to coagulate, forming the curds and whey which are so essential in the cheesemaking process. Humans have been working with rennet for thousands of years, and it is typically readily available in stores which carry cheesemaking supplies; it can also be made at home, if you happen to have access to the necessary ingredients. For vegetarians and kosher Jews, non-animal alternatives to rennet are available.

There is a great deal of mythology surrounding the history of cheesemaking, because humans have been making cheese for a very long time, and the steps involved are actually fairly complicated. The stomachs of ruminants have historically been used to make bags and sacks, and food historians theorize that someone must have stored milk in one a bit too long, allowing it to curdle, and someone thought of turning the curdled milk into a food product. Modern rennet is created through an extraction process which yields neat, dry tablets or a liquid which is very easy to work with.

Traditional rennet was made by washing the stomach of a young ruminant after it has been slaughtered, and then salting it. The salted stomach is kept in dried form, with cooks snipping off small pieces and soaking them in water when they have a need for rennet. Some cheesemakers continue to make and use rennet in this way; the vast majority use commercially processed rennet, which is made by creating a slurry and then subjecting it to a compound which will cause the enzymes to precipitate out.

The main enzyme in rennet is rennin, although there are a few other enzymes as well, and the precise content depends on the animal the rennet comes from; sheep rennet, for example, is different from cow rennet. When added to milk, the enzyme causes the milk to coagulate, essentially starting the digestion process. Once curds have formed, cheesemakers can cut the curds, drain them, and pack them into molds to make cheese.

Several plants produce natural rennet compounds, as do some microbes, and these non-animal sources of rennet can be found for sale in stores which cater to vegetarians, and in shops which produce kosher dairy products. If you are a vegetarian, you may want to be aware that cheesemakers are not required to disclose the source of their rennet, so unless a cheese is specifically labeled as vegetarian, it may contain rennet. Under Jewish dietary laws, milk and meat cannot be mixed together, so cheese which is certified as kosher or pareve will not contain rennet. It is also possible to find cheeses which have been coagulated with acids like lemon juice; paneer is one such cheese.