Good nutrition is one of the most fundamental requirements for all animals and has been identified by Australian livestock farmers as the biggest contributor to animal welfare on farms. Ruminant livestock utilize their extensive fermentation capacity in their forestomach to digest coarse roughages that could not provide adequate nutrients for the welfare of monogastric animals. They are therefore most often kept on rangelands, with a diverse range of feeds of low quality and characterized by a large seasonal variability in growth and quality. However, the variation in feed quality and quantity can be partly buffered by the ability of domesticated ruminant livestock to store food energy as fat tissue, for example in the hump of Bos indicus cattle and fat tail of sheep, to be utilised when feed availability is low. It is important to determine when ruminant livestock are malnourished so that corrective action can be taken. We propose that this occurs when normal functioning of the animal, including behaviour, physiology and reproduction, is adversely affected by an inadequate supply of nutrients. The sensation of hunger is central to the concept that an animal suffers during malnutrition but is also adaptive to motivate the animal to locate the necessary nutrients. It is concluded that a better understanding of malnutrition in ruminant livestock is essential to maintain high welfare standards.