This review addresses the effects of probiotic bacteria on immune development and the role in the treatment and prevention of allergic disease. Although there is a sound theoretical basis for anticipating benefits, there are currently insufficient data to recommend probiotics as a part of standard therapy in any allergic conditions. Furthermore, although there have been several studies to show a benefit in prevention of atopic eczema, other studies have failed to support this. None of the studies has shown any clear preventive effect on sensitization, nor any allergic disease other than eczema. The term "probiotic" is often used loosely to include bacterial strains with little documented immunomodulatory capacity or controlled studies to support the claims. It is not known whether effects in experimental systems have any clinical relevance. Finally, very little is known about this large, complex internal ecosystem. Explanations for the varied results between studies include host factors (including genetic differences in microbial responses and allergic predisposition) and other environmental factors, such as general microbial burden, individual microbiota, diet (including consumption of prebiotic substances), and treatment with antibiotics. As more studies are completed, these factors are likely to make robust meta-analyses problematic to perform.