Productivity in livestock systems is closely correlated with gross efficiency. Improving efficiency by increasing productivity is an honourable pursuit since it minimises waste; not least the waste of animals through disease, infertility or simply the inability to provide food in excess of maintenance. By this (limited) definition the limits to productivity in any system are defined only by the quality of feed available and the genetic and physiological capacity of the animal to convert it into food for man. However, the approach to the limit is asymptotic, i.e. subject to the law of diminishing returns. Until recently most Animal Science has been directed towards marginal improvements in productivity but we now recognise the need to broaden our objectives to include quality of life criteria, such as environmental protection and animal welfare. My remit is to consider how to incorporate a proper concern for animal welfare into the assessment of limits to productivity,or more precisely, the cost/benefit analysis of marginal responses to increased intensity of production.