Non destructive investigations have been applied to the Tower of Orologio in San Marco Square in Venice, in arder to perform a structural diagnosis preliminary to a project of conservative restoration. In this paper, the data achieved "in situ" and by laboratory tests are presented. In particular, the preliminary phase of investigation includes magnetometric, endoscopie, soni c and ultrasonic surveys, the jlat-jack and relative humidity tests. Besides diagnostic investigations, measuring instruments have been installed for the monitoring of deformational behaviour ofthe structures with time. The achieved results, are a determinant factor for the evaluation of the strength characteristics and of the stati c conditions of the structures under investigation. l. INTRODUCTION Before the Tower of Orologio was built, Venetians measured the time thanks to the Sant 'Ali pio clock situated a t the corner of the Basilica of San Marco. In 1495, the construction of a new clock was decided and, in order to do so, the ancient edifice raising along the right side of the Square (property of the Church) was cut, realising a building which guaranteed the continuity of the covered walk to the Pellegrino Calle and the connection between the Square and the Marzaria through the arcade. The covered walk to the Pellegrino Calle has been maintained till the 1717 when the two latera l wings of the Tower were sol d to privates w ho closed the access . .. .:';_ ,.:. • . ·:;,~""!.