Allergic diseases are complex and involve a range of environmentalfactors interacting with a susceptible genotype. The familial clusteringof diseases, such as asthma and hay fever, has been recognised for overtwo centuries, but identification of the genetic basis to this had toawait the molecular biological revolution. Estimates of the contributionthat genetic factors make to asthma susceptibility range from 35% to70%. For the majority of allergic diseases, segregation analysis has notidentified a consistent Mendelian pattern of inheritance, which, whencombined with multiple phenotypes and environmental interactions, hasmade identifying candidate genes especially difficult and, at times,controversial. Part of the difficulty has been lack of agreement overphenotype definitions, reduced power of studies to predict linkage andassociation, and, importantly, lack of true heterogeneity betweenpopulations.Despite these difficulties, the last decade has witnessed enormousprogress in this field. Th