Simulation models have been used productively to aid understanding of animal biology and production systems. Emphasis here is on pig growth models, but the principles apply also to other species and other outcomes. Main applications for such models are direct use within a production system, indirect use across production systems and guidance of research. Most useful models are dynamic and mechanistic; most are deterministic but some gain added utility from stochasticity. Several pig growth models have predicted growth rate, feed efficiency and body and carcass composition from inputs, including protein accretion capacity and diet composition and intake. They have been so successful that now attention can be applied elsewhere. Pigs in commercial production fail to achieve their potential growth, presumably because of various stressors, including disease, and the gap between actual and potential growth is a rich target for future simulation models. Models should address activation of the innate immune system separately from clinical disease. They can provide special benefits from prediction of the impacts of several health-promoting technologies, including those in feed.
