Small devices attached to food packs or pallets that reveal whether the product suffered damaging temperature exposure during transport, distribution, or storage. A common job is flagging thawing of frozen foods in transit — an event that can compromise both quality and safety. Most indicators give an irreversible visible signal, typically a colour change triggered when a threshold is breached; other designs use microbial growth responses, applied especially to animal carcasses. Time–temperature integrators accumulate exposure over the journey. These tools give receivers objective, tamper-evident evidence that the cold chain held, supporting acceptance or rejection decisions at goods-in.