Some ex situ methods for removing pollutans from the environment are listed and compared with the methods used for in situ treatment. Active and multifunctional barrier systems and advantages of their possible use for remediation of polluted groundwater are described. Permeable, undersurface reactive barriers for remediation of groundwater pollution and contamination employ a vertical wall of reactive media, installed in a contaminated groundwater flow, to remove or treat groundwater contaminants in situ. The systems show many advantages, such as lower costs and lower impact on the environment.