Thirty mortar samples from the “Leaning Tower” of Pisa were examined. The specimens include mortars from both the first (1173-1178) and second (1272-1278) stages of Tower's construction. The mineralogical, petrographical, chemical and physical data collected on the bulk mortar and its binder show that the famous “Leaning Tower” of Pisa was built through the constant use of a high-quality hydraulic mortar (average compressive strength about 16 N/mm2; average binder SiO2 content about 29%) as the binding agent for the “infill” masonry. Although a great deal of variability exists in the values of each measured property, even amongst samples from the same stage of construction, the averaged values for the foundations and the above-ground structures erected during the two distinct construction stages are highly uniform. The most conspicuous differences lie in the sand content and granulometry, which are respectively greatest and coarsest on the foundation mortars. Chemical and mineralogical data suggest that the Tower mortar was prepared by mixing slaked lime, obtained from an almost pure limestone, with sand from Arno and Serchio Rivers alluvium and a diatomaceous earth, probably quarried at Mt. Amiata, about 180 km to the south of Pisa. It is estimated that the construction of the Tower from the foundations to the top of the 7th storey (the belfry, added to the main structure only around 1365, is not considered here) required the use of about 1400 tons of slaked lime (assuming 60% water content), 1350 tons of sand and 400 tons of diatomite.