Reports highlighting the positive effects of probiotics on the performance of pigs or on in vitro traits are now quite frequent, but the use of probiotics in feed compounds has not been widespread. Prerequisites for the healthy and efficient growth of young pigs are the rapid maturation of the gut mucosa and the mucosa-associated lymphoid tissue, and the formation of a local stable and complex bacterial community. In neonatal pigs, suckling and the maternal environment shape the gut microbiota. Later, when weaning stress causes a transient drop in favourable bacteria, the oral supply of microbes could contribute to re-establish the microbiota balance. Some strains isolated from piglets were tested for their ability to settle in the intestine. After weaning, piglets experience new and often unfavourable bacteria. Probiotics have been investigated to contrast the enteropathogens, owing to their properties (production of antibacterial molecules, competition on adhesion sites, stimulation of immune response, etc.). Data in general show that their oral administration can be favourable or, at least, innocuous. However, two cases are presented here, where a probiotic given to pigs already combating enteropathogens impaired pig health, and this could be explained by their effect on the immune response. A more tolerogenic response of the host is expected when beneficial bacteria directly contrast the pathogens, probiotics are claimed to directly modulate or even activate the immune system. For one probiotic divergent effects on growth and health are presented, and these differences may be due to different experimental details or different starting microbiological environments. Scarce data are available on specific immune responses induced by commensal microbes in pigs, and on the interaction of resident microbiota with orally supplied probiotics. Increased knowledge of the role of commensal microbiota in the gut and in the pig metabolism, helps in selecting the best bacteria and in designing the best feeding strategies for improving the efficacy and the reliability of their oral use.