The compositions of well-dated archaeological glasses from the Northern Adriatic have been determined to elucidate the origins of the Renaissance Venetian glassmaking industry. Electron probe microanalysis (EPMA) was used to characterize thirty-seven Byzantine-era glass finds from Torcello, an island located five miles to the northwest of the Rialtine islands that make up modern Venice. The Byzantine glass data was used in conjunction with early medieval Torcello glass compositions and late medieval Venetian glass compositions to gain insight into the technological evolution of glassmaking in the lagoon. The three data sets were then examined within the context of archaeological evidence for an early medieval glass furnace complex at Torcello. Our data on the Byzantine glasses reveals that a decline in Roman-style glassmaking technology during this period may have contributed to Venice’s late medieval and Renaissance glassmaking innovations.