Classical Karst is a large area located between the North-Eastern part of Italy and the South- Western part of Slovenia. The area runs for about 40 km long and 15 km wide in the SE-NW direction and is bounded on the NW by the Isonzo River valley, on North and NE by the Vipava River valley, on East by the Pivka River basin, on South and SE bordered by the Cicarija structure while on West is bounded by the Gulf of Trieste washed by the Adriatic Sea. The plateau, slightly inclined towards NW, consists of carbonate lithotypes widely involved by karst landforms. The karst lithological characteristics do not allow the establishment of a perennial hydrography surface due to the high rock mass permeability related to the karstification. Rainfall and river waters are immediately swallowed by the karstified bedrock where they create a network of caves that first transfers the water vertically into the rock mass through the vadose zone and secondly collects them in an aquifer characterized by large horizontal or sub-horizontal conduits which transport the water to the springs area. The main water contribution to the Classical Karst aquifer is well identified in the area where Reka River sinks in Škocjan Cave, in Slovenia; after flowing more then fifty kilometers over impermeable rocks, when the river meets limestones and begins to incise them, it quickly disappears into the caves. From that point it is named Underground Timavo River. The windows opened on the path of underground Timavo River are extremely rare; in Slovenia three deep wells reach large tunnels and rooms flooded by the river (Kacna Abyss, Strsinski dolini Cave and Kanjaduce Cave) while in Italy Timavo River is intercepted by Trebiciano Abyss and Lazzaro Jerko Cave. Few other known cavities are intercepted by groundwater only during the highest floods: these are Claudio Skilan Cave, Massimo Abyss, Gigante Cave, Lindner Cave and Colombi Cave. A deep piezometer reaches the groundwater base level near the limestone-impermeable Flysch contact. The Mathematics and Geosciences Department of Trieste University (D.M.G.) is monitoring all the water points on the Italian Classical Karst territories since 1995; in cooperation with slovenian partners (Park Škocjanske Jame and ZRC SAZU Center of Postojna). In this way, a complete scene on the Classical Karst aquifer is emerging. Only the collaboration between Italy and Slovenia could lead to a reasoned groundwater monitoring network in order to plan the best transboundary aquifer management and protection in order to study the complex karst hydrodynamics.