Capacity-spectrum-based-methods are also used for assessing the vulnerability and risk of existing buildings. Capacity curves are usually obtained by means of nonlinear static analysis. Incremental Dynamic Analysis is another powerful tool based on nonlinear dynamic analysis. This method is similar to the pushover analysis as the input is incrementally increased but it is different as it is based on dynamic analysis. Moreover, it is well known that the uncertainties associated to the structural response can be significant, because of the un-certainties involved in the mechanical properties of the materials, and because the expected seismic actions are also highly uncertain. In this work selected mechanical properties are considered as random variables and the seismic hazard is considered in a probabilistic way. A number of accelerograms of actual European seismic events have been selected in such a way that their response spectra fitted well the response spectra provided by the seismic codes for the zone where the target building is constructed. In this work a fully probabilistic ap-proach is tackled by means of Monte Carlo simulation and it is applied to a detailed study of the seismic response of a reinforced concrete building. The building is representative for multifamily buildings in Spain but the methods used and the results obtained can be extended to other types of buildings. The main purposes of this work are 1) to analyze the differences when static and dynamic techniques are used and 2) to obtain a measure of the uncertainties involved in the assessment of the vulnerability of structures. The results show that static based procedures are somehow conservative and that uncertainties increase with the severity of the seismic actins and with the damage. Low damage state fragility curves have little uncertainty while high damage grades fragility curves show great scattering.