Along with other forms of allergic disease, food allergies appear to be on the increase, with childhood allergies to foods such as peanuts being of particular concern. Around 7?10 foods are responsible for the majority of allergies, including several of plant origin, notably peanut. Allergies are usually triggered by the protein components in a food, which are also known as allergens. However, not all the proteins in an allergenic food like peanut are allergens. Why should this be? This question has been addressed by an EU-funded inter-disciplinary network of clinicians, food chemists and plant biochemists called Protall. From the groups considerations it is clear that, whilst the abundance of a protein in a food is one factor involved in determining its allergenic potential, this is not sufficient on its own to predict its allergenicity. Through an analysis of common properties of plant food allergens that trigger a reaction via the gastrointestinal tract it has become evident that they belong almost exclusively to three structurally related protein superfamilies??the prolamin superfamily (which includes the prolamin storage proteins of cereals, non specific lipid transfer proteins, alpha-amylase inhibitors, and 2S albumins), the cupin superfamily (specifically the 11S and 7S globulin storage proteins) and the cysteine proteases. Whilst we cannot as yet predict the allergenicity of a given protein, such an understanding of the structural attributes of proteins that predispose them to becoming allergens is important if we are to understand what makes some foods more allergenic than others. It is also highly relevant to developing more reliable means of assessing the allergenic risks posed by novel foods, developing effective processing strategies for reducing allergen loads in foods and developing novel therapeutic options as an alternative to dietary exclusion.