Nutrient requirement change over time and individual variability in pigs influences the efficiency of nutrient utilization. These variabilities should be considered to predict nutrient requirements more accurately. The goal of precision feeding is to develop systems able to estimate and deliver, at the right time, a ration with a quantity and composition adapted to the daily requirements of each animal. It would improve feed and nutrient efficiency, which is a major issue for the sustainability of all pig production systems. The objectives of this review were: 1) to define feed efficiency and present the factors that affect it, as well as challenges to and strategies for improving it; 2) to define precision feeding and the sources of variability in nutrient requirementsand show the need for new technology to obtain real-time data; and 3) to present current models and applications of precision feeding for fattening pigs and sows. Feed efficiency is expressed as the ratio of mean daily weight gain to mean daily feed consumption over a given period. In practice, the inverse of this ratio is generally used for breeding animals and represents the efficiency of converting feed into weight gain (feed conversion ratio, FCR). Several factors influence FCR, such as spillage, feed digestibility, composition of weight gain, feed intake and nutrient utilization. Selecting the appropriate form of feed and the appropriate nutrient density and supply, as well as reducing negative effects of environmental factors should improve FCR. New feeding technologies (e.g. sensors, feeders) allow group-housed animals to be fed based on their individual requirements, which improves group efficiency. Predictive models of nutrient requirements and excretion, such as InraPorc, have been developed and used to select the best feeding strategies. For growing pigs, precision feeding strategies are a promising solution to reduce nutrient excretion by adjusting the nutrient supply to each individual at different points in time. Recent simulations indicate that precision feeding might also be a relevant strategy for sows.