Hydraulic cements are energy-cheap relative to other common materials and this was the driving force that led to the development of macrodefect-free (MDF) cements in 1981. In this paper the scientific principles of such materials in terms of porosity, particle packing and rheology are demonstrated and the use of a range of different cement types will be reviewed. The role of the polymeric rheological aid is emphasized and distinguished from that of so-called super-plasticizers used with conventional cements. The distinction between MDF cements and polymer concretes of various types is highlighted and the similarity of MDF cements and chemically bonded ceramics (CBCS) when the polymer is removed is noted. Various properties of MDF cements are given with emphasis on their high degree of mechanical reliability. These properties place MDF cements in a unique position in the materials field, a position that makes the replacement of hitherto more advanced engineering materials, such as metals, polymers and ceramics, a viable proposition. Areas of use are discussed.