A simple mechanistic model of animal growth based on a representation of energy dynamics is presented. The model formulates an animal which generates its own demand for energy (according its environment, genetics, and demands placed on it), and performs according the energy supplied to it. By using the same formulation for both fetal and post-natal stages, the model gives a continuous description of growth from conception to maturity, and allows the same genetic parameters to govern all stages of development. Thus it can be used to study the effects of changes to different selection pathways on growth and its efficiency.