This Technical Note discusses the studies conducted by ESDU for the CFD validation of the predictions for pressure loss coefficient and flow characteristics in square-edged orifice plates. These studies were carried out in support of the update and extension of the correlations developed in ESDU 81039 and used the validation data from the literature reviewed in ESDU TN 07007. Square-edged orifice plates are found in different engineering applications@ such as flow-meters@ fuel lines@ hydraulic systems@ air conditioning systems@ and water pipe systems. The purpose of this work was to generate validated CFD predictions and provide guidance on the CFD modelling of the pressure loss coefficient and flow characteristics in: ? square-edged and thin knife-edged orifice plates@ free of burrs and other contaminants@ ? circular@ concentric orifices in circular pipes with identical downstream and upstream cross-sectional areas@ ? fully-developed flow conditions upstream and downstream of the orifice@ ? swirl-free@ steady flow conditions with no cross-flow upstream and downstream of the orifice plate and flow normal to the orifice plate@ ? laminar@ transitional and turbulent flow regimes@ and ? single-phase@ Newtonian liquids and gases (Mach number ??0.3 at the vena contracta plane) in incompressible flow. The CFD validation studies presented cover the following geometrical configurations and pipe Reynolds numbers: ? diameter ratios: 0.3 ??????0.7; orifice thickness to diameter ratio: 0.01 ??t/d ??5.0. ? laminar@ transitional and turbulent flow regimes: 10 ??ReD ??105. Based on the comparisons of the ESDU CFD predictions with the experimental data in the literature@ best practice guidelines are provided for the modelling of the flow characteristics and pressure loss in square-edge orifice plates.