During the past three decades, urban groundwater has emerged as one of the world's most pressing issues. Explosive population growth, most prevalent in cities, has placed an inordinate demand on groundwater supply, prompting concerns for its long-term sustainability at a time when the quality of available groundwater resources is being increasingly degraded by anthropogenic activity. Cities less reliant on groundwater for potable supply are equally obliged to manage subsurface water with cautious respect since rising groundwater levels can generate a myriad of problems such as unstable land slopes, flooded basements, tunnels and electrical utilities, and the release of polluted water to urban wetlands, springs and streams. "Challenges in Urban Groundwater" is premised on a growing recognition that most urban groundwater problems are not uniquely associated with any particular region or hydrogeological environment, and much can be learned by understanding the successes and failures of others. It showcases the best urban groundwater papers presented at the International Geological Congress held in Florence, Italy in 2004, and is supplemented by contributions solicited from other world experts active in urban groundwater research. Topics covered range from the urban water balance and rising groundwater levels to groundwater contamination and the role of aquifer modelling.