An aquifer is an underground layer of water-bearing permeable rock or unconsolidated materials (gravel, sand, silt, or clay) from which groundwater can be usefully extracted using a water well. The authors of this book present important data on aquifers such as: hydrogeological studies of various dolomite aquifers in Slovenia; (GIS)groundwater modelling as an integrated tool for sustainable management of groundwater resources under changing environments; investigating different groundwater systems occurring in Brazil; an analysis of past catastrophic hydrogeological events (landslides and floods) and their affect on groundwater and aquifers and how to mitigate their damage.