This special section of Urban Water is devoted to the interaction between the urban environment and groundwater. `Paradoxically, urban groundwater is both a valuable asset and a difficult problem'. Potential problems include rising groundwater levels threatening the stability of deep foundations, land subsidence, or simply the slowing of progress of deep tunnel excavations. Groundwater is also a precious resource to be protected for future use. It is under threat particularly by the process of urbanisation. This can reduce natural recharge while at the same time creating new artificial sources of recharge, which can have an adverse effect on groundwater quality.