Current understanding of medieval glass composition is compaerd with the instruction for the preparation and use of glass outlined by Theophilus in the early twelfth century. The points are illustrated by analyses of glasses from a range of objects and locations. They include the Westminster Retable, the pavement at Cluny Cathedral, Romanesque Mosan enamels on copper, Abbey Windows from San Vincenzo in Italy and early enamelled glass vessels which ar likely to have been made in Venice.