Asthma was clinically described in antiquity, and to this day the diagnosis is made on the basis of clinical history, examination, and pulmonary function testing. Throughout the centuries, clinicians have observed that the condition presents in distinct patterns, or phenotypes, suggesting that asthma is not a single disease entity, but many diseases with a common pathway that leads to breathlessness and wheezing. Genotyping offers the promise to help define the diseases of asthma at a basic level, and the potential for molecular diagnostic tests. The emerging discipline of pharmacogenomics ought to assist clinicians in prescribing therapy with optimal efficacy and minimal side effects, and knowledge of mechanisms should assist clinicians in monitoring the outcome of therapy. The study of asthma “subtypes” or phenotypes will focus further genomic research, define genetic subtypes of asthma, and simplify the study of the various individual diseases that produce the symptoms of asthma.