Severe asthma is diagnosed in patients with refractory asthma that is difficult to control despite a thorough reevaluation of the diagnosis and management and after more than 6 months of follow-up by a physician specializing in asthma. Respiratory function measurements and reversibility and bronchial provocation tests (except when contraindicated) are essential in the reevaluation of an asthma diagnosis. Factors that can influence asthma control, such as environmental exposures, comorbid conditions, treatment adherence, and in particular inhalation technique must be recognized and considered appropriately before confirming the diagnosis of severe asthma. Objective criteria are important in the follow-up evaluation of severe asthma. They must include the measurement of some disease components. During follow-up, it is recommended that patients be monitored by validated questionnaires about quality of life and asthma control and by pulmonary function measurements, airway inflammation assessment, and recording the frequency of exacerbations and healthcare system use (including whether planned or unexpected). Severe asthma is a heterogeneous condition that includes several phenotypes. Determining the phenotype of each case of severe asthma will improve our understanding of its underlying mechanisms, natural history and prognosis, will help guide the choice of current and future treatments and will provide useful indications for new therapeutic interventions.