Probiotics have the potential to improve human health, and to prevent and treat a wide variety of diseases. Results from human clinical trials and scientific studies have confirmed the preventive and therapeutic effects of selected strains of microbes in viral and bacterial intestinal infections, and in positively influencing immunological parameters. However, several of these documented results need more rigorous research to be confirmed. Furthermore, it must be remembered that not all Lactobacillus or Bifidobacterium species are equal, and not all over-the-counter products contain the bacterial species listed on the label or, indeed, any viable bacteria at all. Thus, until major improvements occur in the regulation of labelling and quality assurance procedures for probiotic compounds, it is difficult to recommend which products to purchase. In addition, more adequately designed and properly executed clinical trials are necessary to carefully explore and characterize the therapeutic applications of probiotics.