Extensive analysis of disruptions in JET has enabled an advance in the understanding of disruption generated runaway electrons. Tomographic reconstruction of the soft X-ray emission has made possible a detailed observation of the magnetic flux geometry evolution during disruptions. With the aid of soft and hard X-ray diagnostics the runaway electrons have been detected at the very beginning of disruptions. A study of runaway electron parameters has shown that an approximate upper bound for the conversion efficiency of pre-disruptive plasma currents into runaways is about 60% over a wide range of the plasma currents in JET. Runaway generation has been simulated with a test particle model in order to verify the results of experimental data analysis and to obtain the background for extrapolation of the existing results onto larger devices like ITER. It was found that close agreement between the modelling results and experimental data could be achieved if in the calculations the post-disruption plasma electron temperature was assumed equal to 10eV and if the plasma column geometry evolution is taken into account in calculations. The experimental trends and numerical simulations show that runaway electrons are a critical issue for ITER and, therefore, the development of mitigation methods, which suppress runaway generation, is an essential task.