Temperate grass-legume mixes (i.e. pasture) are nutritionally well-balanced and can sustain moderate levels of milk production in dairy cattle. However, pasture is perishable, and if not consumed by cow, it is wasted and the feed quality of the re-growth declines. Moreover, increased use of supplementary feed also increases production costs. This means that cows should only be fed non-pasture feeds when there is insufficient pasture and supplements will have a significant effect on milk production. For good nutrition management in grazing systems, it is therefore essential to identify genuine feed deficits, so as to ensure that the marginal milk production response to providing a supplementary feed is sufficiently large to return a profit to the farmer. This chapter offers a review of the considerations that must be taken into account when deciding whether and how to supplement pasture with additional feed.