Among poultry scientists there is an increasing interes in model building because models are useful in the investigation of the economic consequences of management decisions and in identifying gaps in current knowledge of the biological process involved in production. In addition to empirical formulae giving the relationship between one or two dependent and one or more independent variables, more mechanistic models are needed to solve problems which involve more detailed 'what if' questions, in particular, those that take into account interactions between several factors influencing growth and require a fundamental knowledge of growth processes. The advantage of these models is that they can be used in a broader range of circumstances than empirical models. The development of such mechanistic models serves to identify several remaining gaps in our fundamental knowledge of the causal mechanisms of growth. The mechanistic models described in this review can be seen as a collection of several empirical models, each representing a part of the total biological growth process.