Built in the 6th century at the order of Emperor Justinian, the Monastery of Saint Catherine in the Sinai desert preserves the oldest Christian monastic library with an uninterrupted history. This Greek Orthodox Monastery houses one of the largest collections of manuscripts in the world, holding over 4500 codices in total. These include a significant number of palimpsest manuscripts (over 170) in which eleven different languages are attested in the erased layers as scriptiones inferiores: they reflect the long history and the multicultural nature of the Sinai collection. This book lies at the intersection of palimpsest studies and the investigation of the Byzantine Greek Euchologion (prayer book), with an additional focus on the history of the Sinai library. It offers the first full inventory of the Greek palimpsests preserved at the Monastery of Saint Catherine including a list of newly identified membra disiecta sinaitica. The second part contains the detailed description and historical analysis of three selected Sinai Euchologia (Sin. gr. 960, Sin. gr. 962, Sin. gr. 966) written on recycled parchment. These descriptions are based on original work with the manuscripts, which have never before been studied with regard to their scriptiones inferiores. This study thus offers new insights into the history and development of the Sinai collection of manuscripts over the centuries.