In this paper, published and unpublished information on excretion by dairy cattle, beef cattle and sheep is reviewed. A number of factors are known to affect both the amount ana N content of livestock excreta, most notably animal liveweight, diet and water intake and, for adults, whether in lactation or not. Relationships between liveweight and the volume and N output of excreta have been used to derive estimates for ＇standard＇ outputs for a range of adult and young stock; in the case of breeding stock, allowing for the lactation period. An alternative approach for estimating N excretion, via nitrogen balance calculations, was undertaken for a number of livestock categories and provides some validation of the standards, increasing confidence in their application. These standards are now incorporated into the guidelines already in place for Nitrate Vulnerable Zones in England and Wares and in the recently revised Code of Good Agricultural Practice for the Protection of Water.