The World Health Organization has defined probiotics as “Live microorganisms which, when administered in adequate amounts, confer a health benefit to the host.” Traditionally, probiotic microorganisms (mainly Lactobacillus ssp. and Bifidobacterium ssp.) have been used to prevent or treat diseases in the gastrointestinal tract. In the past 20 years, there has been an increased interest in possible oral health effects of probiotics. In vitro studies have shown promising results with growth inhibition of mutans streptococci (MS) and Candida albicans. There are only a few clinical studies with caries development as the primary outcome while more studies have been focusing on control of caries risk factors or so-called surrogate outcomes. Several studies have evaluated the effects of probiotic bacteria on MS in saliva and/or plaque, and a number of probiotic strains show ability to reduce the number of MS. Probiotic bacteria have not been shown to permanently colonize the oral cavity; in early-in-life interventions or in subjects with a mature microbiota. To date investigated strains are transiently present in saliva during and shortly after an intervention. There are eight randomized controlled clinical trials with dental caries as outcome and probiotic strains, administration, duration of the intervention, and target group varied. In a majority of the studies (75%), the interventions resulted in caries reduction in the treatment groups. Although a majority of these studies suggest a caries-preventive effect of probiotic bacteria, more long-term clinical studies are needed in this field before probiotics could be recommended for preventing or treating dental caries.