Silica aerogel is an open-pored porous transparent material with optical and thermal properties that makes the material very interesting as an insulation material in windows. A number of different aerogels have been investigated for their optical and thermal performance. High thermal resistance of aerogel was found for all the investigated samples and the samples showed very high solar as well as light transmittance. However, all the investigated aerogel samples showed a tendency to scatter the transmitted light, resulting in a reduced optical quality when the aerogels are integrated in glazings. This phenomenon is considered as being the main obstacle to incorporating the material in clear glazings, but a significant improvement of the optical quality of aerogel has been observed during the last five years. A number of prototypical evacuated 500 x 500 x 28 mm(3) aerogel double glazed units employing a new edge seal technique were manufactured and characterized for their optical and thermal properties. As expected, the same scattering of light was found in the aerogel glazings as in the aerogel samples, but excellent thermal performance was found, indicating a glazing type that, from a thermal point of view, is without competition in heating dominated climates.