The tuffaceous building stones of ancient Rome come from quarries in about eight deposits which were erupted from nearby Monti Sabatini and Colli Albani volcanoes between 561 KA and 37 KA. Roman builders and architects employed these well- to poorly-lithified tuffs as building materials in dimension stone and concrete construction. They acquired an excellent knowledge of the diverse behaviors of the tuffs and travertine (from nearby Tivoli) over many centuries of use. To understand how the petrographic and mineralogic assemblages of each tuff lithology influence its strength and durability we measured uniaxial compressive strength (C) under dry, water-soaked, and humid (RH98%) conditions as well as water absorption (Ab) and adsorption at RH98% (Ad) of tuff samples collected from Roman quarries. Point counts of more than 25 thin sections reveal the relative abundances of lava lithic fragments, crystals, vitric fragments, clay, and zeolite (philipsite, chabasite, and/or analcime) and calcite cement. The most durable tuffs (C_dry=31 MPa to 44 MPa) contain >40% lava lithics and primary crystals. Lava lithic fragments provide a weight-bearing framework of dense particles to which zeolite cements may strongly adhere. The least durable tuffs (C_dry=20 MPa to 29 MPa) contain >20% altered vitric matrix and/or >5% pumice fragments. The zeolite cements, formed by alteration of glass and leucite, bind poorly to clay-bearing pumice fragments and altered vitric matrix. Overall, the tuffs are not particularly durable building stones. The Romans applied stucco or fastened decorative marble or travertine cladding on their exterior surfaces. Burial of structures helped to preserve the stone. The high porosity (Ab=11-24%; Ad=3-7%) of the tuffs coupled with their direct exposure to rain and urban weathering at present makes them especially vulnerable to decay. The monuments should be placed under protective cover and the building stones selectively reinforced with consolidants adapted to their particular petrologic and mineralogic charcteristics.