The human gut is an intensively colonized organ containing microorganisms that can be health-promoting or pathogenic. This feature led to the development of functional foods aiming to fortify the former category at the expense of the latter. Since long, cultured products, including probiotics fortification, have been used for humans as live microbial feed additions. This review presents some of the microbes used as probiotics and discusses how supplementation with probiotics may help initiate and/or restore eubiotic composition of gut microbiota. Additionally, it considers safety and regulatory aspects of probiotics.