Asthma typically can range from a disease of very mild and intermittent symptoms to one of debilitating and life-threatening disease. However, the factors that control these differences remain poorly understood. It is likely that the factors controlling the severity of asthma are many and dependent on each other. Potential factors include: (i) inflammatory; (ii) structural; (iii) hereditary/congenital; (iv) environmental; and (v) psychological/emotional; each potentially interacting and influencing the other. Finally, the question remains as to whether all asthmatics, no matter what their initial severity, potentially can develop severe disease, or whether only a certain subgroup will become severe.