About 18% of the total water resources potential of Turkey is made up of groundwater resources. Significant portion of the streamflow of major rivers is supplied by groundwater through springs and baseflow. In 1960s and early 1970s, the financial capacity of Turkey did not allow construction of large dams for irrigation. Development of groundwater resources in alluvial plain aquifers where the agriculture was concentrated has been a priority. In 1990s, the building of large dams has been boosted and irrigation by surface waters preferred due to the lower operational cost. From 1990s, not enough funds have been allocated to explore and develop groundwater resources. In spite of its strategic significance, much more has been invested to investigate and develop the "visible" resource. This unbalanced policy of water resources management has reflected also in the organizational and institutional structure of Turkey. Groundwater resources of Turkey mainly occur in alluvial and karstic aquifers. Large coastal plains and deltas, grabens and pull-apart basins constitute the major alluvial aquifers. The thick and extensive carbonate rocks along the Taurus mountain belt favor formation of productive karst aquifers. The fractured rock aquifers are either low yield or of local importance. Igneous rocks have no permeability and they have very limited outcrops. Groundwater occurs in younger volcanic rocks with limited extension. However, volcanic rock aquifers at foothills of volcanoes, such as Erciyes and Nemrut, may supply a great amount of groundwater where they are recharged by snowmelt. Metamorphic rocks are hydrogeological barriers, in general. They may bear very little amounts of groundwater that might support aquatic ecosystems. Turkey has faced some water mismanagement problems whose consequences are observable in terms of the decline of groundwater levels, reduced spring and streamflows, desiccation of lakes and wetlands and loss of ecosystems. These consequences resulting mainly from managing surface waters and groundwater resources separately, ignoring that they are interacting subsystems of the same and single source, are becoming more frequent and severe. Implementation of the EU-Water Framework Directive has helped, to a certain extent, to maintain the "good status" and to "recover" the degraded water resources and the ecosystems. The "safe yield" approach that has been used in groundwater management needs to be changed to a "sustainable yield" approach which considers also the ecological water needs. This can only be achieved by competent persons who are educated in hydrogeological characterization, conceptualization and modelling of groundwater systems.