During repair work to the south transept of Gloucester Cathedral carried out in 1979-1983, a survey was made of the whole of the south elevation (including the Great South Window, the earliest extant example of the Perpendicular style). Examination of some 5,500 stones revealed that a great number of the original Norman stones had been reused in various positions during a major program of conservation dated by documentary evidence to 1331-1336, which was designed to modernize the poorly lit eastern arm and transepts. The structural problems involved included taking down the south and west walls of the transept to one-third of their original height while keeping the southwest turret intact, and dismantling and re-erecting the high-pitched gable between the two staircase turrets during the insertion of the new window. The structural problems and the solutions adopted are discussed, and a probable building sequence is suggested. -- AATA