Modelling theory and methodology. Definitions developed by discussing examples of simple, illustrative conceptual models. Relationship of models to the scientific process. The (mathematical) modelling process. Methods and tools - with M Bowers. Evolution of current models used in support of animal production. Animal energetic models. Protein and amino acid models. Fasting heat production and maintenance requirements. Models of ruminant digestion. Ruminant digestion. Static models of ruminant digestion. Dynamic models of ruminant digestion. Models of growth. The growth process. Model evaluations of concepts of growth. Mechanistic, dynamic models of steer growth. Lactation. The lactation process. Models of lactating rats. Lactating cow models. Tissue models. Adipose tissue. Liver and visceral metabolism. Mammary metabolism. Speculations. Future uses of research models. On the future use of dynamic, mechanistic models in animal production. Appendices: data tables; derivations of stoichiometric relationships; models.