Probiotics and prebiotics present regulators with challenges because they require a demonstrated positive health outcome and proof that the prebiotic or probiotic is the agent of action once safety aspects have been satisfied. Thus, probiotic and prebiotic definitions are important because they will set the criteria by which these materials will be judged within the regulatory sphere. Use of the terms probiotic and prebiotic are, themselves, considered health claims in some jurisdictions, so that both product health claims and product content labeling may be regulated. Currently accepted definitions of prebiotic and probiotic make it easier to draw a straight line between ingestion and health outcome for probiotics but much more difficult for prebiotics, where a health outcome must be linked to changes in specific bacterial species within the gut microbial community. These challenges highlight the difficulties facing regulatory bodies and the scientific community when emerging science is turned into consumable product.