This thesis focuses on one important component of the energy system - the window. Windows are installed in buildings mainly to create visual contact with the surroundings and to let in daylight, and should also be heat and sound insulating. This thesis covers four important aspects of windows: antireflection and switchable coatings, energy simulations and optical measurements. Energy simulations have been used to compare different windows and also to estimate the performance of smart or switchable windows, whose transmittance can be regulated. The results from this thesis show the potential of the emerging technology of smart windows, not only from a daylight and an energy perspective, but also for comfort and well-being. The importance of a well functioning control system for such windows, is pointed out. To fulfill all requirements of modern windows, they often have two or more panes. Each glass surface leads to reflection of light and therefore less daylight is transmitted. It is therefore of interest to find ways to increase the transmittance. In this thesis antireflection coatings, similar to those found on eye-glasses and LCD screens, have been investigated. For large area applications such as windows, it is necessary to use techniques which can easily be adapted to large scale manufacturing at low cost. Such a technique is dip-coating in a sol-gel of porous silica. Antireflection coatings have been deposited on glass and plastic materials to study both visual and energy performance and it has been shown that antireflection coatings increase the transmittance of windows without negatively affecting the thermal insulation and the energy efficiency. Optical measurements are important for quantifying product properties for comparisons and evaluations. It is important that new measurement routines are simple and applicable to standard commercial instruments. Different systematic error sources for optical measurements of patterned light diffusing samples using spectrophotometers with integrating spheres have been investigated and some suggestions are made for how to avoid such errors.