In this review we focus on dietary fat content and subsequent effects on asthma. According to the World Health Organisation over 300 million people currently have asthma. The majority of asthma cases are 'extrinsic' and result from inappropriate 'allergic' immune responses to inhaled environmental substances. Whilst some individuals are allergic to particular food components it is becoming clear that the content of the diet can more generally affect the health of the immune system. Components of maternal and early life diets have been reported to influence offspring immune function and asthma. There has been speculation that different types of dietary fat have pro- and anti-inflammatory effects but the results of various studies are contradictory. Asthma and obesity are two conditions that have almost simultaneously reached epidemic levels in some societies. There is evidence that diet-induced obesity alters immune function and there is little doubt that consumption of a high caloric diet with high fat content leads to obesity. However, there is conflicting information over whether and how obesity is linked to asthma in children and adults. Whilst obesity is to be avoided there is accumulating evidence that dietary fat per se does not necessarily predispose towards allergic symptoms.