Irritable bowel syndrome (IBS) affects a significant proportion of the North American population; however, the etiology and pathophysiology of this disease remain poorly understood, and treatment is focused on symptom management. Over the years, research has revealed that the bacterial flora in the human gut interact with the bowel in a complex and dynamic relationship and may be responsible for the induction and progression of some of the pathophysiologic aspects of IBS. Probiotics are nonpathogenic bacteria that benefit the host, and the roles they can play in the bacterio-gut relationship provide hope of a safe treatment that would allow for modulation of IBS disease states. Probiotic treatment for IBS has undergone significant exploration, yet the exact therapeutic effects and doses of these beneficial bacteria remain unclear due to the conflicting nature of available evidence. This review discusses the evidence from randomized controlled trials on probiotic treatment of IBS and presents the current understanding of the mechanisms of action of probiotics as they apply to IBS and provides a plausible explanation for the variability in evidence documented by the various trials under review.