Summary Dietary supplements and special diets that purportedly enhance cognitive function are widely promoted in many countries. Nutritional interventions claiming to relieve stress and depression, increase energy or improve memory are common. Limited regulatory requirements for marketing dietary supplements or special diets, particularly in comparison with drugs, place substantial responsibility on scientists conducting research in this area. Agreement on scientific standards for the conduct and evaluation of research is clearly desirable, so that scientists can achieve consensus regarding such claims. Many of the methods used to assess dietary supplements and other nutritional factors are similar to those used in other fields, such as psychopharmacology, and are widely accepted by investigators conducting nutrition and behaviour research. Key methods in common include use of double-blind procedures, dose–response studies and placebo treatments. Some of these procedures can be difficult or impossible to implement in nutritional studies. Other critical methods, such as selection of appropriate tests to assess cognitive performance and mood state, are more controversial among scientists working in this and other areas, and difficult to standardise. Scientific consensus on these issues will ensure availability of safe and efficacious dietary supplements and help eliminate dangerous or ineffective products.