The biochemistry of ensiling is essentially a simple process, which, however, can become complex when interactions among plant enzymes and the activities of numerous microbial species become involved. The desired effect is the conversion of simple plant sugars such as glucose and fructose to lactic acid by lactic acid bacteria (LAB) in an anaerobic fermentation. When sufficient lactic acid has been produced, all microbial activity is suppressed, primarily through the effect of undissociated lactic acid, and the silage can then be stored anaerobically until required for feeding. Complications arise because: 1. There are always aerobic periods at the start and end of the ensiling process. 2. Simple sugars are not the only substrates metabolized. 3. Plant enzymes and other microbial species apart from LAB compete for substrate. The complexity of ensilage is increased further when the difficulties of controlling large-scale processes like silage making on-farm are also considered.