It is widely accepted that in protected cultivation cover materials significantly affect crop growth, yield and quality. Hence, cover materials play an important role in the design and construction of greenhouses/screenhouses. This article reviews issues and opportunities related to cover materials and presents industry perspectives. A literature survey shows that various greenhouse cover materials (glass, rigid and flexible plastic sheets and screens) are used in different regions of the world to cover greenhouses and screenhouses. The various materials differ in their physical, chemical, mechanical and radiometric properties. Selection of the type of cover material by growers depends on many factors and is mainly affected by the local tradition, which is related to the geographical climate and solar radiation intensity, the crop value, the cost of the cover material and its life span. Over the years cover materials have been improved to enable a higher light transmittance, a change in the characteristics of the transmitted light (e.g. change the percentage of diffuse light within the greenhouse), a reduction of heat loss and hence higher energy saving and prevention of condensation and droplet formation on the inner surface of the cover. Furthermore, properties of cover materials were changed to enable reduction in insect invasion into greenhouses/screenhouses on one hand and improve orientation of beneficial insects in the house on the other hand. Attempts have also been made to reduce development of fungal disease on the crop by changing the spectrum of light transmitted into the greenhouse. Nowadays, different glass surface treatments or suitable cover materials, although sometimes a combination of both, are in the vicinity to be used in order to increase the uniformity of the greenhouse climate and enhance plant growth. For example, diffused glass is usually created by treating the surface of low-iron glass to create patterns that scatter the light. In addition, diffused glass, as well as covering diffused film, may have irregular wavy microstructure on the surface and capable of altering the optical properties (haze and light transmission) according to requirements for different climatic regions. Next to that, micro structures are combined with nanostructures, such as anti-reflection treatments (AR-glass), and also, hydrophilic film with anti-fog additives that can be used in order to improve light transmission into the greenhouse. Recently, solar cells were incorporated into cover materials to generate electricity and semi transparent plastic sheets that can generate electricity were developed and tested. In addition to the topics mentioned above, this article considers the deterioration of cover materials due to different environmental effects and discusses the resultant changes in light transmission and characteristics. It also presents methods to measure the properties of different cover materials.