The development of the gastrointestinal tract is a very sophisticated process, which starts during prenatal life and continues after birth. Diet is the most important factor modulating structure and functions of the intestine. In the early post-natal period the major role play bioactive substances of colostrum and milk. Their properties influence the dynamics of apoptosis and mitosis and through gastrointestinal tract development they influence animal adaptation to solid feed. The weaning period is associated with dynamic intestinal microflora development, which activity also exert a significant effect on processes taking place in the intestine. Gut function during the development and the response to feed component are often evaluated on the basis of changes in the structure of intestinal mucosa and enzyme activities. The degree of intestinal maturation influence nutrient digestibility, feed efficiency and immunity, thus greatly affecting animal production.