The introduction by the Task Group's convenor L. Taerwe 'Model uncertainties in reliability formats for concrete structures' gives an outline of the general approach summing-up his former contribution to CEB Bulletin 219 'Safety and Performance Concepts' on the consistent treatment of model uncertainties in reliability formats for concrete structures. The second contribution 'An analysis of model uncertainties: ultimata limit state of buckling' by M. Pinglot, F. Duprat and M. Lorrain investigates the model uncertainties of hinged columns and the influence of boundary conditions and proposes appropriate safety elements. The third contribution 'Model uncertainties concerning design equations for the shear capacity of concrete members without shear reinforcement' by G. König and J. Fischer compares suggested formula from various sources (CEB-FIP Model Code, Eurocode 2,Remmel) to 176 test results from a data base covering concrete strengths from 20 to 111 MPa.