People have exploited fermentation by lactobacilli for centuries as a means of preparing and preserving foods. Several different bacterial species are today used as probiotic bacteria in different products both for human and animal applications. By definition probiotic bacteria are “live microorganisms which when administered in adequate amounts confer a health benefit on the host”. The most commonly used bacteria for the probiotic concept are found within the lactic acid bacteria (LAB) group. One of several genera included in the LAB group is Lactobacillus. One species of Lactobacillus, Lactobacillus reuteri has been studied extensively and certain strains have been shown to be probiotic i.e. health promoting with diverse beneficial effects, and thus it was interesting to further investigate the properties of these bacteria. To put this thesis work into context, the field of probiotic research is described and examples of proven probiotic effects are discussed. The overall aim was to investigate L. reuteri and its microbial action in the microbiota of humans and its relationship to health and disease. L. reuteri was shown to be a natural component of human milk. It was found in approximately one in seven nursing mothers living in geographically widely separated countries. Breast milk may be considered as a natural synbiotic and evidence from these results suggest that L. reuteri is one of the beneficial components in this regard. L. reuteri was also proven to be effective in reducing both gingivitis and dental plaque in patients with moderate to severe gingivitis, suggesting an improvement in periodontal health. Bacterial antagonism through the probiotic administration in the oral cavity might have contributed to the observed alleviation of symptoms and clinical manifestations of periodontal disease. It was shown that L. reuteri supplementation of pregnant mothers and their offspring during the first year of life resulted in detection of L. reuteri in breast milk and infant stool. Administration of L. reuteri resulted in the presence of L. reuteri in saliva, but no significant effect on supra- or subgingival microbiota was observed. The significant increase in plaque index in the control group with no significant change in the test group may however indicate a probiotic effect of L. reuteri in this study population of healthy individuals.