Near infra-red (NIR) analysis has been developed over the last 20 years into a routine method for many agricultural commodities and food constituents. The theoretical and statistical framework, which has made this possible, is reviewed and the extent of the applications presented. The development of a NIR method is demonstrated and the difficulties and limitations of the method are discussed. On-line methods of using NIR analysis are being developed and this is expected to be one of the most important applications for the future use of NIR in the food industry.