Introduces Behavior3D, a flexible concept for declarative modelling of 3D object behaviours. Contributes a node concept based on X3D with object-oriented features such as inheritance, strong typing, and polymorphism. The behaviour nodes can be used alongside built-in X3D nodes as first class scene graph elements. A rich set of predefined behaviours is proposed, among them Animation and State Machine node collections. The concepts were successfully implemented with VRML97/X3D and simplify the definition of rich 3D behaviours and animations in Web3D scenes. The work was presented at the premiere event in the Web3D community, and is cited by 28 (Google Scholar). RAE_import_type : Conference contribution RAE_uoa_type : Computer Science and Informatics