In this paper, the most critical physical properties for the greenhouse covering materials are reviewed. The physical properties considered include the radiometric properties (transmissivity, reflectivity and absorptivity) over several radiation bands and also the K value (the overall heat transfer coefficient). Only a few standard methods for testing these physical properties of covering materials for greenhouses exist. Usually, some of the testing methods used for plastics in general, are also applied to greenhouse covers despite important functional differences. These testing methods are reviewed with regard to their suitability for application to greenhouse covering materials. Also, reported values of the most important physical properties of greenhouse covering materials are presented and reviewed. The values presented are found to vary considerably among the materials considered. These variations are attributed mainly to the testing method employed and also to the type of the material. Existing testing methods are used arbitrarily and technical information is presented in a rather confusing way. Modification of existing testing methods is required because, in most cases, they are insufficient for greenhouse covering materials. Also, new testing methods should be elaborated and specially designed for greenhouse covering materials.