The present paper presents an overview of the work of the RILEM Technical Committee 159-ETC on the engineering of the interfacial transition zone in cementitious composites. The object of this committee was to resolve the practical influences of the interfacial transition zone (ITZ) in cementitious composites on their engineering properties. The major issues highlighted by the committee are: (i) The ITZ should not be viewed as a well defined material property but rather as a system property which is dependent on the overall composition as well as the method of fabrication of the cement composite. (ii) The structure of the ITZ should be quantified experimentally and by modeling in terms of gradients of microstructure. (iii) The quantification of the structure and mechanical properties of the ITZ is based on either scaling up of the interface by making special composite specimens or by in-situ testing of the actual composite by micro-level techniques. The former can be quantified more readily, but the interface which its specimens represent may be quite different than the one in the real composite. (iv) Inverse modeling can provide a strategy for determining the properties of the ITZ by testing of the whole composite and modeling its behavior. (v) In concretes the characteristics of the ITZ may have a moderate influence on mechanical properties but not a drastic one. (vi) The ITZ may have a drastic effect on the mechanical properties of fiber reinforced cement composites and on their long term properties.