This chapter describes key factors that determine the health efficacy for humans of foods containing probiotic lactic acid bacteria. The published data on human studies (including properly designed clinical trials) with the most successful commercial probiotic lactobacilli are summarized. Conclusions are drawn as to which health or disease conditions can be alleviated or cured by probiotic lactobacilli. The chapter describes health conditions in which the proof is strong, in which uncertainties exist, and in which there may be possibilities in the future.