In ruminants near infrared spectroscopy of faeces (F.NIRS) can measure the concentrations of faecal constituents such as N and fibre. It can also directly estimate many diet attributes, including total N, fibre, digestibility and the major plant groups (e.g. monocots versus dicots, and some plant species). In some situations F.NIRS can be used to estimate voluntary intake and liveweight change, but it is difficult to encompass animal effects (e.g. lactation, maturity), pasture availability or mineral deficiencies. Application of F.NIRS, especially in conjunction with other measurements (e.g. metabolizable energy intake calculated from liveweight change, 13C/12C ratio in faeces, microbial protein synthesis and phosphorus concentrations in faeces) can provide valuable and reliable information about the nutritional status of grazing ruminants. Examples where F.NIRS has been used to measure the nutritional status of grazing cattle and sheep are described for extensive and intensive tropical production systems in northern Australia, the Caribbean, the Amazonian humid tropics, and for temperate pastures in Europe. F.NIRS can be applied to routinely and economically measure the nutrient intakes of grazing ruminants, and such information used to modify herd management.