This review article is a summary of a seminar organised by the European Respiratory Society on "The role of small airways in obstructive airway diseases" which was held in October 2010 in Amsterdam, the Netherlands. The aims of the seminar were to identify important questions related to small airways involvement in asthma and chronic obstructive pulmonary disease (COPD), and to discuss future approaches based on current and evolving knowledge. Data obtained by pathological and physiological measurements in small airways and their relevance to clinical manifestations and therapeutics in asthma and COPD were reviewed. It was concluded that our knowledge on the roles of small airways in asthma and COPD is limited. Studies of large numbers of well-characterised subjects using multiple methods (genetic characterisation, cell biology and physiology, imaging) and integration of the data using mathematical models are suggested to be of interest. The availability of these techniques coupled with our ability to better target inhaled molecules to small airways provide a unique opportunity for a reappraisal of the relevance of small airways in chronic airway diseases.