Thermal-hydraulic aspects of valves used in nuclear power plants are discussed. Emphasis is given to a review of recent work on safety and pressure relief valves, including the presentation of some experimental results describing one- and two-phase flow through such valves. Measurements of internal flow fields are presented. Significant flow separation occurs in both the safety and the pressure relief valves. An experimental study using high-speed photography to determine vaporization sites in safety valves when the flowing medium is initially a compressed liquid is described. A new method of estimating the observed decrease in valve coefficient when choked vapor flow changes to liquid flow is developed. This method also permits the prediction of the observed dependence of the valve coefficient on the receiver pressure in the choked and unchoked flow regimes. The fluid dynamic phenomena that lead to valve disk vibrations are discussed.