Generation of RunAway Electrons (RAEs) is often observed at disruptions in JET. RAEs are usually detected with the soft and hard X-ray and neutron diagnostics. At the current quench stage a large population of the high-energy RAEs (typically with several MeV) can often create a current plateau achieving sometimes more than 50% of the pre-disruptive plasma currents. For the future reactor-scale devices this phenomenon constitutes a serious problem since the localized deposition of several Mega-Amperes multi-MeV RAEs current to the components of the first wall may cause severe damage to the device. This paper presents recent progress on studies of the disruption generated runaways and their main dependencies on plasma parameters and disruption conditions. The evolution of the primary RAEs generated at the thermal quench and their role in further development of the runaway process has been examined. It is found that primary runaway beam can exist at an early stage of the disruptions even with strong re-arrangement of the magnetic structure. Runaway current plateaus were usually not detected at disruptions with a relatively high electron temperature (Te~100eV) immediately at the beginning of the current quench phase. There are indications that auxiliary plasma heating also may affect the creation of continuous plateau, even in cases when hard X-rays and neutron emission have been observed. Data on disruption generated RAEs obtained in JET prior and after divertor installation has been analysed in order to study possible trends for runaway generation during disruptions at the nominal experimental parameters in ITER.