Strength characteristics of cement based composites are affected by their microstructure: stronger cements have a much lower porosity which is often determined by their initial water/cement ratio. Conventionally cements are brittle materials with low tensile strength, but despite this, are widely used in construction. High strength cements are even more susceptible to catastrophic brittle failure than conventional cements, mainly because their (slightly) higher tensile strength leads to release of higher stored energy at failure. The work in this thesis follows an approach previously taken by members of the Chemistry and Engineering Departments at Aberdeen University to promote toughening in conventional cement pastes. The toughening mechanism depends strongly on the nature of the interface between angular inclusions and the continuous cement matrix. The situation is more complicated in high performance cements because the matrix is more compacted so that controlling interfacial properties is more difficult. Pore reduced cement and silica fume blended cement are used as model high strength matrices. The increased density of conventional high strength pastes that results from the lower water cement ratio and better particle packing does not immediately lend itself to this toughening mechanism because of the expected increase in physical bond between matrix and aggregate. Despite these fears, increases in strength and fracture toughness have been achieved using both high strength matrices with the angular debonded inclusion toughening mechanism. The present study has also shown that the measured toughness can be used as a means of assessing the relative strength of the matrix angular aggregate interaction. In order to manufacture tougher materials it has been shown that the matrix should be stronger.