The discussed topic of this final paper is "Energy efficient windows". In order to achieve energy-efficient windows, it is necessary to know its advantages and disadvantages in order to choose a material that will meet the requirements of energy efficiency without disturbing the external appearance of the building. Designed and installed windows must be functional and durable. The most important requirement of energy efficiency is thermal insulation. With the choice of materials, it is necessary to satisfy protection against unnecessary energy loss in winter, but also protection against excessive heating in summer. Windows are most often made of wood, aluminum and PVC, but there are also combinations of them. Glass can be two-layer, threelayer or four-layer. The glasses are often coated with various coatings, and the space between the glasses is filled with gas. For a high-quality performance of windows, the final installation with air- and water-impermeable materials is essential, which ensures a continuous envelope of the building resistant to atmospheric influences and energy losses. It is necessary to achieve the appropriate security level of protection against burglary with locks. This paper provides an overview of the theory of the energy balance of windows, but also a presentation of common technologies for the production of frames, glass and glass edges.