The role of the agricultural scientist is to manipulate crop and livestock response phenomena so that they serve society's needs better, enabling countries to obtain full benefit from their agricultural resources. By producing food more efficiently, resources can be made available for non-agricultural development and other needs beyond the essentials of food and fibre. This text provides an introductory outline of the analytical principles involved in appraising the efficiency of crop-fertilizer and livestock-feed response. It provides students of both agricultural science and economics with a simple but formal exposition of the why, how and wherefore of the principles of crop and livestock analysis, thereby helping to further co-operative effort among biological and economic researchers. The third edition has been updated and revised, with additions relating to the principles of modelling, the concept of economic duality as pertinent to response processes, the appraisal of aggregate response, and the economics of response research.