Self-heating food packaging is active packaging with the ability to heat food contents without external heat sources or power, usually using an exothermic chemical reaction. Packets can also be self-cooling. These packages are useful for military operations, during natural disasters, or whenever conventional cooking is not available. They are often used for military field rations, camping food, instant food, or other types of food intended for preparation where proper cooking facilities or methods are unavailable or less ideal.

The source of the heat for the self-heated can is an exothermic reaction that the user initiates by pressing on the bottom of the can. The can is manufactured as a triple-walled container. A container for the beverage is surrounded by a container of the heating agent separated from a container of water by a thin breakable membrane. When the user pushes on the bottom of the can, a rod pierces the membrane, allowing the water and heating agent to mix. The resulting reaction releases heat and thus warms the beverage which it is surrounding.

The heating agent and responsible reaction vary from product to product. Calcium oxide is used in the following reaction:

Copper sulfate and powdered zinc can also be used, but this process is less efficient:

Anhydrous calcium chloride is often used as well. In this case, no chemical reaction occurs, instead the heat of solution is generated.

Commercial heat sources for self-heating food packaging use an exothermic (heat releasing) reaction, for which there are several common formulations. These include:

Some newer formulations use a Thermite-like reaction between a more reactive metal powder such as aluminum or magnesium, with a less reactive metal oxide such as iron oxide or silicon dioxide.

During World War II Heinz took a different approach, using a mechanism developed by Imperial Chemical Industries which involved igniting a smokeless fuel in the can.