The uncertainty assessment for the development of the new seismic hazard maps of Spain is explained in this work. A detailed description of how uncertainties are considered in the different phases followed for the probabilistic seismic hazard assessment study is presented. Special emphasis is given to the characterization of the uncertainty of earthquake size parameters contained in the catalog, and their propagation to subsequent phases of the analysis such as catalog homogenization, completeness and declustering. Aleatory and epistemic uncertainties are examined and assessed using Montecarlo simulations and a logic tree, respectively. The final impact of uncertainty in hazard estimates is evaluated through coefficient of variation maps, showing the areas where larger and lower variability is obtained. 2 The hazard maps developed in this work are intended to be the basis for the future revision of the Spanish seismic code.