A dynamic model of beef cattle growth and composition was developed based upon concepts validated in detailed modelling analyses of the growth of individual organs. Concepts that cell number (DNA) and cell size are primary determinants of organ (animal) size are included. Initial body weight (BW) and composition (percentage body fat), mature body weight, feed intake and feed net energy values for maintenance (NEM) and growth (NEG) are inputs to the model. Alternative inputs descriptive of feed can be metabolizable energy (ME) concentration and efficiencies of ME use for maintenance and fattening. Current body DNA and protein content and ME intake are primary determinants of rates of protein accretion. Net synthesis of body fat is calculated after accounting costs of maintenance and protein synthesis. Rate constants defining DNA synthesis, and protein degradation and synthesis, and effects of feed additives and implants upon these were deduced using data from comparative slaughter (energy balance) trials with over 1,000 cattle (mainly Hereford) fed mixed diets. No systematic errors in prediction of body weight or composition due to mature body weight, ration or feed intake (FI) were evident within this data set.