The major part of the Swedish hydro power development took place between 1945 and1975. At that point of time, significant progress had been made regarding the use of highquality concrete for hydraulic structures. In spite of the progress made, deterioration ofconcrete due to freezing and thawing is still frequent. Superficial damage at the waterlineand spalling of concrete far below the water level on the upstream face of concrete damshave been observed. Knowledge about the deterioration processes is important in order toimprove the efficiency of maintenance of hydro power plants. Results obtained from twoexperimental studies of frost damage mechanisms are presented in this paper.