This paper presents the results of a detailed CFD (computational fluid dynamics) analysis of the performance of a range of reversing flow air turbines for use as a power take-off system in Oscillating Water Column (OWC) Wave Energy Conversion (WEC) systems. In the first part of the paper a fundamental CFD analysis of the lift and drag data for various aerofoil profiles, using a number of different turbulence models, is described and quantitative comparisons are made with a range of experimental data previously published by other researchers. The wide variation in experimental data lift coefficients for nominally identical aerofoil profiles are discussed. The second part of the paper then describes CFD results of the performance of various rotors of air turbines. Of particular interest is the “interference effect” whereby the lift and drag data of a blade in a rotor, or cascade, is different from that of a single, isolated aerofoil. The interference effect is important when blade-element methods are used to determine the performance of air turbines, particularly for optimal sizing of the turbine during design stages. The present CFD results are compared to earlier analytical inviscid flow analysis of the aerodynamic performance of cascades for the first time, and the implications for blade element analysis of turbines are discussed.