The recent changes in trade barriers between countries are permitting more exchange of goods across borders with reduced difficulties. Fenestration products (windows, doors and skylights or roof-mounted windows) are among the popular construction products to cross boundaries. In many cases. fenestration manufacturers have to demonstrate that their products meet certain performance criteria wherever the products are sold. There exist national and international standards for the evaluation of the performance requirements of fenestration products. Usually, the demonstration of compliance to performance requirements involves extensive product testing programs which, in many cases, becomes a major financial burden on the producers. The most common performance requirement is the thermal transmission coefficient (U-factor) of the fenestration system. In recent years, there have been major efforts to harmonize fenestration performance standards in order to reduce the financial burden on the product manufacturers and ultimately on the consumers. In particular. the standards for the determination of the thermal characteristics of fenestration systems are among those that are gaining major attention by manufacturers, building regulators and standards writing organizations, This paper presents an overview of some national and international standards, which mainly deal with the determination of the thermal transmission coefficient, U-factor, of fenestration products.