Three approaches for the evaluation of uncertainty expected in calculations performed by system thermal-hydraulic codes are discussed : a) The statistical approach initially proposed by United States Nuclear Regulatory Commission, later on made robust by GRS (Geselschaft fur Reaktosicherhet) in Germany with the introduction of the Wilks’ formula. The approach is also known as propagation of code input errors. b) The deterministic approach based on the propagation of code output error, including the methodologies UMAE (Uncertainty Method based on the Accuracy Extrapolation) and the CIAU (Code with capability of Internal Assessment of Uncertainty), proposed by UNIPI (University of Pisa). c) The third approach is based on ASAP (Adjoint Sensitivity Analysis Procedure) and GASAP (Global Adjoint Sensitivity Analysis Procedure) initially proposed by Cacuci D. and later on extended by Petruzzi and D’Auria (UNIPI) including concepts from DAA (Data Adjustment and Assimilation). The three approaches are discussed here for possible interaction with uncertainty methods proposed within the Computational Fluid Dynamics (CFD) community.