The responses of poultry (broilers, laying hens and broiler breeders) to nutrients, discussed here, are in economically important outputs such as body weight (or protein) gain, breast meat yield, egg output, food intake and conversion efficiency, numbers of chicks produced per hen, etc. Of interest are the (usually) curvilinear responses of populations (flocks), which are the result of integrating the responses of the individuals making up that population. Populations of birds therefore do not have ‘requirements’ for nutrients: what nutritionists seek are the optimum economic dietary contents of each nutrient, and for this they need to know how populations respond to increasing dietary contents of the essential nutrients. Descriptions of such responses, whilst taking account of the marginal costs and revenues, are therefore invaluable in determining how to maximise or minimise the objective function chosen for any given commercial operation. Clearly, being able to predict these nutrient responses may be seen as the foundation of a successful nutritionist.