Probiotics are a therapeutic class being increasingly used for a variety of GI disorders. Probiotics appear to alter intestinal microflora and may exert their effect(s) by a variety of mechanisms. The GI tract plays an important role as an interface between the host and the environment. It is colonized by about 10 trillion microbes of many different species, amounting to 1–2 kg in weight [O’Hara and Shanahan, 2006]. Potential mechanisms of actions of probiotics include: (1) modulation of GI immunity by altering inflammatory cytokine profiles and downregulating proinflammatory cascades or inducing regulatory mechanisms in a strain-specific manner; (2) displacement of gas-producing, bile salt-deconjugating bacterial species and thus possibly inhibiting pathogenic bacterial adherence; (3) alteration of bacterial flora by acidification of the colon by nutrient fermentation; (4) enhancement of epithelial barrier function; (5) induction of µ-opioid and cannabinoid receptors in intestinal epithelial cells; (6) reduction of visceral hypersensitivity, spinal afferent traffic, and stress response.