Atopic (allergic) asthma is the most common disease of childhood and is strongly genetic in origin. Many genome-wide screens for asthma and its associated traits have now been carried out, and genetic linkage has been consistently identified in several regions. It is probable that these loci contain major genes influencing atopy and asthma. Candidate genes have already been identified from the cytokine cluster on chromosome 5 and the MHC on chromosome 6. These complex regions contain more than one susceptibility locus for allergic disease. Other regions do not contain obvious candidate genes, and positional cloning of these loci is likely to identify novel disease pathways. Parent-of-origin effects are prominent at some of the loci and some also show linkage to other inflammatory immune diseases. Several single gene disorders are associated with allergic disease and on occasion are also linked to the same chromosomal regions. The positional cloning of asthma genes is now feasible.