Horticultural products such as fresh fruit and vegetables are still metabolically active when harvested. Whereas under certain conditions some anabolic processes such as photosynthesis and synthesis of flavor components may still take place, the major metabolic processes are of a catabolic nature and include respiration and fermentation but also other processes such as cell wall degradation. Respiration and fermentation are of particular interest to postharvest technologists because they are strongly associated with quality-associated processes taking place in fruit and vegetables. As a consequence, the reduction of the respiratory activity of horticultural products has been a major concern in postharvest technology. Appropriate refrigeration procedures, often in combination with the application of controlled atmosphere storage conditions, have therefore been developed to reduce the respiration rate and are applied nowadays on a large scale.