There is now unequivocal evidence that dietary behaviour is related to illness and risk of chronic diseases such as cardiovascular disease and cancer. Attempts to improve the nation's diet are based on providing information, assuming that given more information, the public will choose healthier diets. Many studies indicate, however, that nutrition knowledge has little association with dietary behaviour; but a review of the literature reveals that nutrition knowledge has been inadequately measured. In addition, dietary behaviour has been assessed in terms of food intake and not in relation to changes in, or readiness to change, food intake. Following the Introduction, this research begins, in Chapter 2, by reviewing the literature measuring nutrition knowledge. It is found that while many studies measure knowledge, typically the measure forms only part of the study which assesses either a particular subpopulation or a particular aspect of nutrition. In consequence, questionnaires are designed for a one-off and specific purpose and little attention is paid to the psychometric properties of the instrument. Dietary behaviour is measured with one of the well-established methods of assessing intake, the problems of which are acknowledged in the literature. Chapter 3 describes these methods with their shortcomings and use in psychological research. In response to these reviews, a comprehensive nutrition knowledge questionnaire was developed (in 1994) and intake was conceptualised in terms of dietary change, in keeping with psychologists' role in nutrition. Following the development and pilot study of this questionnaire (Chapter 4), its validity and reliability were assessed further in Chapter 5, with positive results. Significant differences were found between criterion groups (dietetic and computer science students), providing evidence of construct validity. Internal consistency correlations ranged from 0.50 to 0.92 and test-retest reliability correlations ranged from 0.80 to 0.98. This measure was then used (Chapter 6) to assess the level of nutrition knowledge among a large representative sample of British adults in a postal survey (in 1995). Nutrition knowledge was found to be poor concerning the dietary recommendations for meat, starchy foods, fruit and vegetables; the different types of fat (saturated, poly- and monounsaturated); and associations between diet and diseases, such as fruit and vegetables, heart disease and cancer. Both stages of change (using Prochaska and DiClemente's model) and consumption of fat, fruit and vegetables (to test the stages' validity) were also assessed as measures of dietary behaviour. Most respondents replied that they had been limiting their fat intake for more than 6 months, but not been thinking of increasing their fruit and vegetable intake. Multivariate analyses showed that being female, having more educational qualifications and being in a higher socioeconomic class were predictive of knowing more about nutrition and having a healthier dietary behaviour. Relationships between nutrition knowledge, stages of change and dietary intake were examined in Chapter 7 and significant associations identified. In contrast to this cross-sectional research, the final study in Chapter 8 was longitudinal and examined changes in nutrition knowledge and dietary behaviour over a one-year period (from 1993 to 1994). This study aimed to provide information on the extent to which healthier changes in dietary intake are related to increases in nutrition knowledge. While changes occurred in dietary intake (fat and sugar intake decreased significantly, the increases in fruit and vegetable consumption were insignificant), knowledge scores remained unchanged. The final chapter discusses the key findings of this research, its implications and areas worthy of future investigation. For example, the results from this research suggest that knowledge is an important factor in food choice and should not be discounted as a part of health promotion. It may also be useful to integrate the construct of knowledge into the social cognition models of dietary choice or indeed to develop a new model to include knowledge along with motivational constructs from the social cognition models.