Model testing in wave tanks or under sheltered sea conditions is an essential step in the development of wave energy converters. The paper focuses on the rules for geometric, hydrodynamic, thermodynamic and aerodynamic similarity in model testing of wave energy converters of oscillating-water-column (OWC) type, with emphasis on air compressibility effects in the air chamber and on air turbine aerodynamics. It is shown that the correct volume scale ratio for the air chamber is far from identical to the volume scale ratio for the submerged part of the converter, and should take into account the thermodynamics of the compressible flow through the air turbine or through the turbine simulator (orifice or other). For those cases when the model is large enough to be fitted with a scaled air turbine, dimensional analysis is applied to obtain ratios for turbine size and rotational speed, and also to establish relationships between rotational speed control algorithms. A numerical example is presented to illustrate the importance of appropriately simulating the air compressibility effects when testing at model scale.