The present knowledge of the pozzolanic material properties is summarized and some problems not yet solved concerning the pozzolanic activity are underlined. The structure complexity and the wide variability of chemical and mineralogical composition of pozzolanas justify the difficulties which arise in establishing general validity relations between chemical and physical characteristics and activity of pozzolanas. The possibility of determining the active phase composition is assessed by testing three different methods: chemical analysis of pozzolana and its insoluble residue, chemical analysis of the parts separated by sedimentation in heavy liquids, electron microprobe analysis. Finally the main properties of pozzolanic cements are examined and compared with those of portland cements. The higher durability of concretes made with pozzolanic cements is underlined but it is emphasized that this important characteristic has not yet been exhaustively explained.