Questionnaires are often used in clinical and nutritional research to subjectively evaluate the effect of an intervention on an outcome measure or e.g. to select a study population with specific characteristics from a larger pool of potential study participants. Nevertheless, their thoughtless application in health claim-targeted studies have frequently resulted in EFSA dismissing the outcomes generated by these questionnaires. Validation is key in the application of questionnaires. But don’t forget, even in the case of validated questionnaires being available, the question remains justifiable whether the purported validation is also valid for your ingredient (e.g. in terms of effect size) and/or your target population. Therefore, even when using generally recognized validated questionnaires, it may be wise to validate the questionnaire in a pilot study or to include some form of internal validation of the questionnaire in the full, health claim-targeted RCT.