ABSTRACT: Since 1920s, first maps showing hydrogeological features on the territory of Slovakia were drawn by mapping geologists, mostly to point out the most important springs - sources of potable groundwater, thermal or mineral water. Since 1950s, maps were produced to accompany the results of massive regional hydrogeological investigations. The whole area of Slovakia was covered by the uniformly created basic hydrogeological maps in 1:200,000 scale, accompanied by the maps of groundwater chemistry in the same scale at the end of 1970s. The content of basic hydrogeological maps at 1:50,000 covering Slovakia was gradually developed since 1991. The basic characteristics of aquifers - transmisivity and its variability, groundwater outflow, lithology and stratigraphy are expressed by background colour, by intensity of colour, number (index) and hatching. Background colour represents the average value of aquifer transmisivity. Hydrogeological maps are gradually being transferred to geographical information systems, and published electronically in HTML format.