All published research examining effects of humor and laughter on physical health is reviewed. Potential causal mechanisms and methodological issues are discussed. Laboratory experiments have shown some effects of exposure to comedy on several components of immunity, although the findings are inconsistent and most of the studies have methodological problems. There is also some evidence of analgesic effects of exposure to comedy, although similar findings are obtained with negative emotions. Few significant correlations have been found between trait measures of humor and immunity, pain tolerance, or self-reported illness symptoms. There is also little evidence of stress-moderating effects of humor on physical health variables and no evidence of increased longevity with greater humor. More rigorous and theoretically informed research is needed before firm conclusions can be drawn about possible health benefits of humor and laughter.