Attention was paid to the levels of importance of maintenance, protein and fat deposition in growing monogastrics. Some information was presented on the composition of their daily gain. Theoretical efficiency values of fat synthesis were derived from biochemical information and compared with those found in balance trials. Factors determining maintenance needs including thermoregulation and physical activity were discussed. Some aspects of energy metabolism and regulation of body fat deposition and mobilization were described including the processes of hyperplasia and hypertrophy. Finally, the role of fat as a well-regulated, rapidly adaptable buffer within the mono-gastric’s total metabolism was emphasized.