Spores are dormant, highly resistant forms produced by bacteria such as Clostridium and Bacillus and by many moulds, designed to survive conditions that kill growing cells. Bacterial endospores survive boiling, drying, and many disinfectants, which is why canning uses retort temperatures above 100 °C and why low-acid foods need full sterilization rather than pasteurization. In the laboratory, spore counts indicate heat-process adequacy and raw-material hygiene, and the heat resistance of spores is quantified as D-values that thermal-process calculations are built on. Respecting spore resistance is the foundation of safe canning and sterilization.