The D value — the decimal reduction time — is the heating time needed at a stated temperature to kill 90% of a microbial population: one log cycle. Every organism has its own D value at each temperature: the D value of Salmonella at 70°C might be seconds, while bacterial spores need far longer. Thermal-process designers use D values to calculate pasteurisation and sterilisation schedules — stacking enough log reductions to reach the required safety margin, then validating with the z-value, which describes how the D value changes with temperature. The concept turns microbial death kinetics into engineering: time-temperature combinations chosen from data rather than guesswork. Canning, milk pasteurisation, and sous-vide safety all rest on D values measured in the laboratory and applied on the line.