Flavour development and detection is, in a chemical sense, dependent upon the interactions of natural products in foods and, in a biological system, reliant upon the interaction with taste receptors. In some cases (for example, capsaicin) the taste molecule is present and is liberated to have an immediate effect. In others a chemical (for example Maillard) or a biological (the ‘mustard bomb’) reaction is required. The correlation between taste, aversion and toxicity has been proposed however it is difficult to determine if there is a quantitative structure activity relationship (QSAR) or not. The response to many xenobiotics appears to be mediated by G-Protein Coupled Receptors (GPCRs) including those responsible for the perception of taste and odour. Some interest has been associated with extra-oral taste receptors and their potential role in mediating non-taste responses. One of the other receptor-mediated responses is vision and the appreciation of colour is an important aspect of foods.