During the excavations of early magnate dwellings, pre-Christian cult sites and early trade centres, several fragments from window glass were found. Until recently, fragments of window glass from pre- Christian sites were most oftenregarded as modern glass or as a form of ‘contamination’ of excavated fields. Our research and analyses show that glass was already being used in the windows of pagan temples and on the estates of the social elite by the early Viking Age. Although glass windows were not common in Viking Age Scandinavia, our research shows that they may have existed on royal estates during this period and in pre-Christian cult sites and temple buildings. It is interesting to note that glass windowsactually appeared in Scandinavia long before they were believed to exist, especially in parts of buildings where they were not used until recently. The presence of window glass in early magnate dwellings, pre-Christian cult sites and early Viking trade centres seems to reveal an interesting pattern. Glass windows in pre-Christian Scandinavia were most probably used in the same way as in early Frankish and Anglo-Saxon churches. This paper presents some results on window glass originating from pre-Christian cult sites in Tissø and Strøby Toftegård (Sealand, Denmark), Sorte Muld (Bornholm, Denmark) and Uppåkra (Scania, Sweden), as well as Viking Age sites in Haithabu (northern Germany), Kaupang (Norway) and Birka (Sweden). This window glass was analysed using laser ablationinductively coupled plasma-mass spectrometry (LA-ICP-MS) at the Centre Ernest-Babelon (CNRS, Orléans, France). The results showed the presence of three main types of glass composition: natron glass, wood ash glass and wood ash–lime glass. If natron glass illustrates the re-use of ancient windows, the presence of wood ash and wood ash–lime glass enabled new light to be shed on the use of early glass windows in southern Scandinavia.