Hand warmers,instant heat pack, boot warmers, etc. are small (mostly disposable) packets which are held in the hand (or stuck to the underwear or socks) and produce heat on demand to warm cold hands. They are commonly used in outdoor activities such as hiking and skiing to keep the extremities warm and assist insulated clothing. They can also be used to heat small household pets during a power outage. Other types of warmers are available to provide soothing heat for muscular or joint aches.

On one side the packets often have adhesive strips so they can be stuck to the underwear (never directly onto the skin as this can cause burns) to keep the lower back warm, for example. There are also types that can be inserted into the shoes or stuck to the socks.

Depending on the type and the source of heat, hand warmers last between 30 minutes (recrystallisation) to 12–24 hours (platinum catalyst).