Probiotic bacteria are defined as live food ingredients that are beneficial to the health of the host. Probiotics occur naturally in fermented food products such as yogurt, kefir, sauerkraut, cabbage kimchee, and soybean-based miso and natto. Numerous health benefits have been attributed to probiotics, including effects on gastrointestinal tract function and diseases, immune function, hyperlipidemia, hypertension, and allergic conditions. A systematic review of the medical literature failed to identify any studies that directly investigated the potential ergogenic effects of probiotics on athletic performance. Two published articles suggest that probiotics may enhance the immune responses of fatigued athletes. In summary, although scientific evidence for an ergogenic effect of probiotics is lacking, probiotics may provide athletes with secondary health benefits that could positively affect athletic performance through enhanced recovery from fatigue, improved immune function, and maintenance of healthy gastrointestinal tract function.