Pasteurization (or pasteurisation) is the process of heating liquids for the purpose of destroying viruses and harmful organisms such as bacteria, protozoa, molds, and yeasts. The process was named after its creator, French chemist and microbiologist Louis Pasteur. The first pasteurisation test was completed by Pasteur and Claude Bernard on April 20, 1862.

Unlike sterilisation, pasteurisation is not intended to kill all micro-organisms (pathogenic) in the food or liquid. Instead, pasteurisation aims to achieve a "logarithmic reduction" in the number of viable organisms, reducing their number so they are unlikely to cause disease (assuming the pasteurised product is refrigerated and consumed before its expiration date). Commercial-scale sterilisation of food is not common, because it adversely affects the taste and quality of the product.

Pasteurisation typically uses temperatures below boiling since at temperatures above the boiling point for milk, casein micelles will high temperature/short time (HTST)]] and Extended Shelf Life (ESL) treatment. Ultra-high temperature (UHT, Also known as Ultra-heat treated) is also used for milk treatment. In the HTST process, milk is forced between metal plates or through pipes heated on the outside by hot water, and is heated to 71.7 °C (161 °F) for 15-20 seconds. UHT processing holds the milk at a temperature of 138 °C (250 °F) for a fraction of a second. ESL milk has a microbial filtration step and lower temperatures than HTST. Milk simply labeled "pasteurised" is usually treated with the HTST method, whereas milk labeled "ultra-pasteurized" or simply "UHT" must be treated with the UHT method.

Pasteurisation methods are usually standardised and controlled by national food safety agencies (such as the USDA in the United States and the Food Standards Agency in the United Kingdom). These agencies require milk to be HTST pasteurised in order to qualify for the "pasteurised" label. There are different standards for different dairy products, depending on the fat content and the intended usage. For example, the pasteurisation standards for cream differ from the standards for fluid milk, and the standards for pasteurising cheese are designed to preserve the phosphatase enzyme, which aids in cutting.

The HTST pasteurisation standard was designed to achieve a 5-log reduction (0.01% of the original) in the number of viable microorganisms in milk. This is considered adequate for destroying almost all yeasts, mold, and common spoilage bacteria and also to ensure adequate destruction of common pathogenic heat-resistant organisms (including particularly Mycobacterium tuberculosis, which causes tuberculosis and Coxiella burnetii, which causes Q fever). HTST pasteurisation processes must be designed so that the milk is heated evenly, and no part of the milk is subject to a shorter time or a lower temperature.