We reviewed existing literature concerning the effects of probiotics on serum cholesterol levels in animals and humans, with particular attention to the possible mechanisms of their action. Probiotics are live microorganisms, which, when administered in adequate amounts, confer a health benefit on the host. One specific benefit that has been reported is that certain probiotic strains (e. g. lactobacilli, bifidobacteria and enterococci) can lower serum cholesterol levels. However, conclusions regarding such hypocholesterollemic effects can vary from studies on animals and humans due to differences in their physiology. As for the cholesterol-lowering mechanisms, different hypotheses have been proposed, including: (I) cholesterol is absorbed into the cellular membrane or cytoplasm; (II) cholesterol is bound to the cellular surface; (III) cholesterol is co-precipitated with free bile acids; (IV) conjugated bile acids were hydrolyzed by probiotics and the resulting free bile acids are more likely than are conjugated ones to be excreted from the body; (V) free bile acids were bound to the cellular surface by capsule exocellular polysaccharides produced by probiotics; (VI) food-derived indigestible carbohydrates were fermented by probiotics to produce propanoic acid in the gut, which can then decrease systemic levels of serum cholesterol by inhibiting hepatic cholesterol synthesis; (VII) a reduction in cholesterol absorption by probiotics through the down-regulation of NPC1L1 gene expression of cells; and (VIII) cholesterol micelle is disrupted by probiotics. The future research is needed to further confirm these hypotheses.