Groundwater remediation using permeable reactive barriers (PRB) can provide a cost- and energy-effective and environmentally friendly technology. The targeted pollutants to be removed from the groundwater by the reactive materials in the permeable barrier are either decomposed to less dangerous compounds or efficiently attached to the reactive material. The most thoroughly studied decontamination processes are chemical reduction, oxidation, precipitation, sorption and biodegradation, while halogenated hydrocarbons, (nitro)aromatic compounds, heavy metals, nitrates, arsenates and phosphates have so far been treated successfully. PRBs are capable of treating groundwater and leachates polluted with uranium compounds or other radionuclides, so the technology is applicable to the remediation of former uranium mining sites. At present it is little known about the long-term behaviour of such systems: changes in porosity and reactivity of the reactive media may impair their long-term functioning. The PEREBAR research project has been initiated within the 5th Framework Programme of EU to look into these issues by developing accelerated testing methods, increasing efficacy by introducing chemical ligands into the reactive material and combining the reactive barrier principle with electrokinetic effects. Bench- and floor-scale testing as well as on-site investigations on a former uranium mining site in southern Hungary will help move the results towards practical application.