Difficult asthma is an uncommon disease (likely < 5% of total asthma). It is poorly understood and often difficult to treat. The first step is to make an accurate differential diagnosis and to avoid the most important factor, which influence the condition of the disease. Between these factors, the exposure to allergens and the gastro-esophageal reflux seems to be the most important. The term is usually applied to conditions that are hard to solve, sometimes dangerous or even life-threatening forms of asthma: corticoid-resistant asthma, corticoid-dependent asthma, brittle asthma, premenstrual asthma and near-fatal asthma.