Non-cystic fibrosis bronchiectasis is a heterogeneous condition and its pathogenesis is still not well defined. A combination of a defect in host defense and bacterial infection allows microbial colonization of the airways resulting in chronic inflammation and lung damage. An ongoing cycle of infection and inflammation may be established. Typically, the walls of the small airway are infiltrated by inflammatory cells causing obstruction whilst mediators, such as proteases released predominantly by neutrophils, damage the large airways resulting in bronchial dilatation. Adjacent parenchyma is also involved in the inflammation. Lung function testing generally demonstrates mild to moderate airflow obstruction that progresses over time. There are a large number of different aetiologic factors associated with bronchiectasis. A variety of different microbial pathogens is involved and they change as disease progresses.