"Norway has more than 100 years of experience in the design and construction of hydropower plants consisting waterway systems that included unlined pressure tunnels and shafts. The waterway systems are in general very long and consist of unlined pressurized headrace tunnels, unlined high-pressure shafts, underground powerhouse caverns, access, and tailrace tunnels. The maximum static head that the unlined pressure tunnel has reached is 1047 meter, which is equivalent to almost 10.5 MPa. This is a world record, and it is obvious that the rock mass in the periphery of unlined pressure tunnels and shafts experience high hydrostatic pressure exerted by the flowing water discharge. Experienced gained from the construction and operation of these unlined pressure tunnels and shafts were the key to develop design criteria and stability assessment principles by giving focus on engineering geology, rock mass quality and geo-tectonic environment. As a result, these criteria and principals have got worldwide acceptance. However, the success of these criteria depends on the engineering geological and geo-tectonic environment prevailing in the are of concern and the operational regime adopted in the hydropower plants. This key-not lecture reviews some of the first attempts of the use of unlined pressure tunnels and shafts concept, highlights major failure cases, discusses the gradual development of design criteria for the unlined pressure tunnels and shafts and highlights recent operational trends that have direct influence on the stability of unlined pressure tunnels and shafts of hydropower plants."