Recent studies on source complexity such as multiple shock nature of large earthquakes are reviewed. Brief notes on the historical background as well as inversion techniques for complex body waves are also included. Through the body wave inversions, complex rupture patterns have so far been obtained for individual earthquake sources. The source complexity is characterized by the following three properties: (1) discreteness of rupture propagation; (2) heterogeneity of stress distribution; (3) hierarchy of rupture elements. These properties suggest the existence of some rate-determining processes which may be associated with ‘ductile’ barriers surrounding ‘brittle’ asperities.