A mechanistic and dynamic model was designed and constructed to simulate beef cattle growth and related body composition for different animal types under various nutritional conditions. This model takes into account major processes involved in the evolution of body composition. We considered proteins and lipids in carcass and non-carcass tissues, which led to a total of four state variables and differential equations. The evolution of these state variables is controlled by synthesis and degradation processes, and depends on the physiological age of the animals and on metabolizable energy supply. Empty and full body weights are deduced from protein and lipid contents through allometric equations. Adaptation processes were considered in order to represent metabolic and gut fill changes that can occur at the beginning of a re-feeding phase after restriction. A total of 26 parameters is needed to run the model. Whenever possible, parameter values were taken from the literature or from our own expertise and measurements. Even so, the value of five of these parameters had to be fitted. An example is given to illustrate the relevance of the basic hypotheses governing the model.