Only by undertaking studies on whole organisms, particularly humans, can we really assess the importance of foods in relation to health benefits. But, undertaking human intervention studies presents the scientist with a number of scientific and ethical dilemmas which are often in conflict. It is not ethically acceptable to undertake a human study which is not scientifically valid, so before designing an intervention study the aim of that study must be clearly defined and then the limitations of any markers of effect or practicality of execution, evaluated in the light of the original aim. Once the science of a study has been determined then the degree of benefit to both the individual and society must be weighed up against the inconvenience or risk for the participant. Nutritional intervention studies should be considered with the same degree of care as pharmaceutical trials, particularly as healthy volunteers are frequently involved who are unlikely to gain any personal benefit from the study. Additionally, when considering the biological activity of phytochemicals, especially those associated with herbal medicines, and with the development of nutraceuticals the line between nutrition and pharmacy is blurred. In Britain, human intervention studies involving healthy volunteers, not conducted within The National Health Service, must still be subject to ethical review and increasingly the use of Hospital Local Research Ethics Committees representing a wide range of professionals is being encouraged by The Department of Health in their guidelines for research governance. The ethical aspects of these guidelines are ultimately based on the Helsinki Declaration. Volunteers who take part in a study must understand the purpose of that study; to this end a full written information sheet must be provided and a verbal explanation given well in advance of the start of the intervention. Only once a volunteer understands the study can his or her consent be considered fully informed and thus have legal status.