Epidemiological studies indicate that most asthma starts in childhood. However, the characteristics of adults with difficult asthma differ from the general asthma population. Difficult asthma as defined by poor control despite high dose inhaled steroids and long acting beta 2 agonists disproportionately affects women, more often has onset in adult life and is less frequently associated with atopy, and therefore appears to be a distinct phenotype. If an alternative definition of difficult asthma that is life threatening asthma is used then many of the behavioural traits may have their origin in childhood. Patients with severe and difficult asthma form a specific subgroup of asthmatic patients with high medical need and research specifically into this group of patients is now needed.