This paper presents three different low energy windows for residential buildings which have been developed at the Technical University of Denmark during the last 8 years. The three windows are designed to maximize the net energy gain. The net energy gain of windows is the solar gain minus the heat loss integrated over the heating season. The solar gain can be calculated for one orientation or averaged over different orientations representing a standard house. Calculation of the net energy gain assumes that all of the solar gain from the windows can be utilized for space heating during the heating season in residential buildings. Problems with overheating in the summer period must be solved with overhang or moveable solar shading devices. To increase the net energy gain slim frame profiles have been developed to increase the glazing area and thereby the solar gain. The challenge when developing slim frame profiles is to make enough space for hinges and fasteners and still maintaining the functionality and strength of the window. Proposals for new hinges and alternative mounting method for windows are also presented in this paper. The first window constructed was made of wood profiles and a low energy double glazing unit. The other two windows were made of fiber-reinforced plastic (plastic reinforced by fine fibers made of glass). This composite material is a weatherproof material with very low thermal conductivity and high mechanical strength. These properties make the material very suitable for frame profiles due to lower heat loss and longer durability of the window. The glazing in these fiber reinforced polyester windows is both unsealed and sealed triple glazing units. The paper also presents the results of a U-value measurement of one of the fiber-reinforced plastic windows which confirms the thermal calculations. The measurement showed a U-value of 1.03 W/m²K for a window in standard size (1480 mm • 1230). The net energy gain for a standard single-light window in Denmark is approximately minus 40-50 kWh/m² per year. The windows presented in this paper have a positive net energy gain of 0 - 20 kWh/m² per year.