Probiotics are live microorganisms that live in gastrointestinal (GI) tract and are beneficial for their hosts and prevent certain diseases. In this chapter, after a complete introduction to probiotics, definition, mechanism of action, and their classification, currently used organisms will be discussed in detail. Moreover, different kinds of nutritional synthetic products of probiotics along with their safety and drug interaction will be noticed. This chapter mentions all clinical trial studies that have been done to evaluate probiotic efficacy with a focus on gastrointestinal diseases.In the end, findings of our pilot study regarding the effect of probiotic on Small Intestinal Bacterial Overgrowth (SIBO) will be presented. The nutritional effects of Probiotics on a host's health will be collected and their usage criteria will be discussed. Some suggestions for the Probiotics daily consumption will be presented and the follow-up for their new adverse reaction will be emphasized, if any.