The beneficial impact of probiotic strains on human health and longevity was noticed over 100 years ago. Since then, microorganisms with probiotic properties have been used in prophylaxis and therapy of many medical conditions, mainly those affecting the gastrointestinal tract but also those caused by immunological disturbances, such as food allergies, atopic dermatitis and reoccurring upper respiratory tract infections. Despite the very wide range of recommendations for probiotic use, many of them are not supported by reliable research confirming medicinal properties of probiotics. The results of randomized trials, published meta-analyses and literature reviews regarding probiotic use in many medical conditions are surveyed in the present work.