[en] Asthma is a complex disease highly dependent of environmental exposure and genetic background. Through linkage analysis, positional cloning and genome wide association studies, novel asthma genes have come out such as ADAM-33 or ORMLD3. Important environmental factors include allergenic exposure, pollutants and especially particulate matters, tobacco, aerosol exposure, viral infections and level of exposure to endotoxin. The effects of environmental factors are modulated by the genetic sequence and numerous single nucleotide polymorphisms (SNPs). Recently, it has also become clear that environmental factors may alter gene expression by DNA methylation or histone methylation/acetylation without changing the gene sequence and thereby changing asthmatic phenotype.