Asthma prevalence and morbidity have increased in the past 10 years in the face of improved knowledge about pathophysiology and treatment. Changing patterns and interactions among asthma risk factors may contribute to these disease trends. Diet is a newly recognized potential risk factor for asthma occurrence. This chapter focuses on the methodological issues in the assessment of diet as a risk factor for asthma and the available data linking diet to asthma, airway inflammation and airway responsiveness, and it concludes with a consideration of research needs and future directions. Four types of dietary constituents are considered: breast feeding and food avoidance in infancy; antioxidant vitamins, specifically vitamin C; dietary cations, specifically sodium and magnesium; and N3-N6 fatty acids. At present, available data are insufficient to implicate any dietary constituent as a causal risk factor for asthma. Data are strongest for vitamin C, which is associated with protective effects of airway responsiveness, lung function and asthma symptoms. Prospective cohort studies of the effects of early childhood diet on the development of asthma in children (birth to age six years) are needed to assess diet as a risk factor for early childhood asthma and its interrelationship with other risk factors.