A comprehensive review of the characteristics, classification, uses and technology of pozzolans was made in connection with a geologic investigation into natural pozzolanic materials in N.-central California. These materials may be siliceous sediments or siliceous pyroclastic rocks, but volcanic tuff and pumice are most frequently used. Vitric tuffs having a high proportion of amorphous silica generally have good pozzolanic properties, whereas crystal and lithic tuffs containing a high proportion of mineral or rock fragments are inferior. These physical properties are readily determined by simple petrographic methods. Tuffs are widely distributed in N.-central California but most of them are lithic or crystal tuffs. Vitric tuff that apparently is suitable for use as pozzolan occurs in the Nomlaki tuff bed which crops out extensively in the western and northern parts of the Sacramento Valley. Vitric tuff also occurs in an unnamed rhyolitic tuff bed that crops out in several small areas near Oroville. Preliminary tests using this tuff in lime-mortar mixes developed sufficient strength to indicate it could be used as pozzolan if properly prepared. The Tuscan Formation warrants further investigation as a possible source of cementitous material.