Lactobacillus is one of the largest genera included in the lactic acid bacteria. This genus contained a lot of species, with their differences in phenotypic and genotypic properties. They are gram-positive, aerotolerant non-spore-forming bacteria. They are commonly found in nature. Additionally, many lactobacillus species constitute a significant part of the human gastrointestinal microbial community and vaginal microbiota. They have a great role in the balance of intestinal microflora. The genus Lactobacillus is the most researched bacteria, which is widely used in probiotic and commercial products. Probiotics are supplements of food containing significant amounts of living microorganisms, and able to alter the composition of the gastrointestinal microbial community, and are used in the treatment of disturbed gastrointestinal microbiota that provides beneficial effects to human health. The mechanisms of probiotic lactobacillus activities include adhesion to intestinal mucosa to prevent the adherence and growth of pathogenic bacteria through competitive exclusion. Also, productions of antimicrobial substances, antiviral activities, antitumor activities, immunomodulatory activities, and maintenance of intestinal balance in certain GIT pathology are methods of promoting host conditions by certain species of this genus.  In this article, many studies that report probiotic activities of specific Lactobacillus species are reviewed. So as to summarize the advantages and benefits that these bacteria promote in the host.