We review the different quantitative trade studies on the expected economic effects of Transatlantic Trade and Investment Partnership (TTIP). Methodologically the studies can be divided into those employing computable general equilibrium (CGE) models and structural gravity (SG) models. Predicted welfare gains vary significantly between the different studies, but these differences can be mainly explained by two components: differences in the expected trade cost reductions and differences in the economic model employed to calculate the welfare effects of these reductions. After a careful assessment of the studies, we conclude that reliable estimates of trade cost reductions are between 3% and 15% and reliable estimates of the welfare effects are between 0.2% and 2% real income gain for the TTIP partners.