Hydropower plants with underground powerhouse contain different types of underground structures namely: diversion tunnels, headrace and tailrace tunnels, access tunnels, caverns grouting and inspection galleries etc. As earthquake ground shaking affects all structures above and below ground at the same time and since some of them must remain operable after the strongest earthquake ground motion, they have to be designed and checked for different types of design earthquakes. The strictest performance criteria apply to spillway and bottom outlet structures, i.e. tunnels, intake, outlet, underground gate structures, etc. In the seismic design of underground structures it must be considered that the earthquake hazard is a multI-hazard, which includes ground shaking, fault movements, mass movements blocking entrances, intakes and outlets, etc. Underground structures for hydropower plants are mainly in rock, therefore underground structures in soil are not discussed in this paper. Special problems are encountered in the pressure tunnels due to hydrodynamic pressures and leakage of lining of damaged pressure tunnels. The seismic design and performance criteria of underground structures of hydropower
plants are discussed in a qualitative way on the basis of the seismic safety criteria applicable to large dams, approved by the International Commission on Large Dams (ICOLD).