In arid and semi-arid regions groundwater is often the only natural resource for water supply. Therefore, stakeholders face great challenges in managing this resource in a responsible way. The problem is further amplified by population growth, increase in agricultural, industrial and municipal water consumption, and the threat of climate change. Therefore, an optimal use and management of the scarce groundwater resources is imperative. A precondition for this is a sound understanding of the particularities of the hydrogeology of arid and semi-arid regions as well as a proper knowledge of the water budget, water resources in storage, and water quality. This book summarizes the results of the conference "Hydrogeology of Arid Environments", which was held in March 2012 in Hannover (Germany). It gives an overview about current research on this topic with examples from arid and semi-arid areas from all over the world. The book is intended for scientists, engineers, hydrologists, hydrogeologists, and political decision makers interested in the water resources of arid and semi-arid environments.