The paper presents a study on the impact of uncertainty on the dynamic response of electric power systems. Three sources of uncertainty are considered, namely, (i) uncertainty in the values of the parameters of physical devices; (ii) uncertainty in the models of dynamic devices; and (iii) variations of the parameters and the numerical scheme to integrate the differential algebraic equations that describe the system. A Monte Carlo analysis is used to define the impact of each source of uncertainty as well as all sources together on the dynamic response of the well-known IEEE 14-bus system.