The simulation, by means of numerical models, of hydraulic transients occurring in the supply of water to hydroelectric or thermal power plants has now become an essential task in the verification of future plant designs, as well as, in the planning of modifications to existing plants. This paper presents the RECAS hydraulic transients simulation code which was developed by ENEL (Italian Electricity Board), to meet the needs of designers of power plant hydraulic systems. This flexible code is able to handle variations in channel geometry and hydraulic network topology, as well as, different types of intake structures. The algorithm incorporated in the code is able to deal with open or closed sections, and allows the study of short high-velocity sections. The code`s validity was confirmed in several applications.