The immune system of infants is actively downregulated during pregnancy and therefore the first months of life represent a period of heightened susceptibility to infection. After birth, there is an age-dependent maturation of the immune system. Exposure to environmental microbial components is suggested to play an important role in the maturation process. The gastrointestinal tract is the major site of interaction between the host immune system and microorganisms, both commensal as well as potentially pathogenic. It is well established that the mammalian immune system is designed to help protect the host from invading microorganisms and other danger signals. However, recent research is emerging in the field of host-microbe interactions showing that commensal microorganisms (microbiota) are most likely one of the drivers of immune development and, in turn the immune system shapes the composition of the microbiota. Specific early microbial exposure of the gut is thought to dramatically reduce the incidence of inflammatory, autoimmune and atopic diseases further fuelling the scientific view that microbial colonisation plays an important role in regulating and fine-tuning the immune system throughout life. Therefore, the use of pre-, pro- and synbiotics may result in a beneficial microbiota composition that might have a pivotal role on the prevention of several important diseases that develop in early life such as necrotizing enterocolitis and atopic eczema.