Grazing as a feeding system is inconstant: there are frequent variations, short-term, middle-term, and long-term ones, in the quantitative and qualitative supply of herbage, which determine the voluntary intake and the performances of the livestock. Hence the advantage of quantifying the effects of the main factors that influence the intake of grazing cattle and sheep. The responses of voluntary intake to animal characteristics and to the feeding value of the forage that have been observed in the manger do mostly also apply to grazing. The amount of ingested herbage increases with the production potential of the animals, and also after a period of restricted feeding. When grazing is intensive, the intake is also regulated by non-nutrition al factors, such as herbage availability (amount on offer, ease of browse). A leafy sward of good digestibility is essential for large voluntary intakes and high individual performances. The laws of responses to intake factors, set up for short-term intervals, are consistent with the results of animal performances measured over the whole grazing season. Taking these laws into account should improve the predictions of the consequences of various practices and management options on animal performances.