The technical and economic results of a broiler flock depend on complex interactions between the animal and nutritional and environmental factors. It is possible to approach this complexity by considering the animal as a component of a mechanistic model, which can represent the diversity of genotypes and farming practices. INAVI simulates broiler growth as a function of nutritional and environmental parameters. The animal is represented by a simplified diagram of its energy balance with few parameters including physical activity expenditures. INAVI is first calibrated with user data (reference curves for feed intake and growth) associated with 'optimal' nutritional and environmental conditions. At each time step (1 h), the actual feed intake is estimated from the comparison (thermostat) between calculated and reference heat production. Inputs change the energy flows inside the simulation submodel using accessible response laws. A user is therefore able to adapt the model to their own data by changing these laws. Simulations illustrating the potential use of INAVI by different stakeholders of the poultry supply chain are presented in this chapter. In that sense, INAVI could be helpful to design new and more sustainable poultry production systems. However, this will require further improvements to the model, and thus stresses the never-ending process of modelling.