The prevalence of asthma and allergic diseases has increased dramatically over the past few decades with the highest incidence occurring in children. Most asthma and related atopic disorders have their origins in early life. Thus, it is imperative to understand the early life origins of the disease in order to identify targets for prevention and early intervention. Although atopic diseases have genetic determinants, the increased incidence of these diseases has occurred far too rapidly for genetic changes to explain the increase. This, most likely, results from changes in environmental influences acting on a pre-existent genetic susceptibility. One of the environmental changes over the last 20–40 years that could have contributed to the recent increase in atopic diseases is diet. Food allergy is often one of the earliest manifestations of atopy, and sensitization to food is a risk factor for the subsequent appearance of respiratory allergy and asthma. However, studies investigating the effects of dietary restrictions on the prevention of allergy have been disappointing. On the other hand, current data suggests that exclusive breastfeeding should be encouraged for at least 4–6 months in infants at both high and low risk of atopy. Increased risk of asthma has also been observed in low birth weight infants, suggesting that under-nutrition can detrimentally alter foetal development. Epidemiological and immunological studies also suggest that dietary modification or supplementation in the foetal and early life could reduce the development of atopic diseases. The current dietary hypotheses relate to antioxidants, lipids, electrolytes and probiotics. The aims of this report are: (i) to assess the best methods to analyse nutrient intake and nutrient status; (ii) to review the existing epidemiological evidence for an association between dietary intake (nutrients and food) and allergic diseases; and (iii) to define the windows of opportunity for nutritional supplementation to be used as a preventative strategy for asthma and allergy.