The rheology of concrete is often approached by using a particle-matrix model, which roughly can be described as coarser aggregate dispersed in a suspension of fines less than 125 mu m. Parameters determining the flow resistance (taken as the area under the shear stress versus shear rate curve) of concrete matrix have been discussed. The following parameters are treated in some detail; water-to-cement ratio (w/c), cement type (including mineralogy and fineness), silica fume dosage and plasticizers (type, dosage and time of addition). It is shown, for instance, that the w/c for a cement fit very well to the so called Krieger-Dougherty equation for suspensions where the apparent viscosity is dominated by the volume fraction of solids. Another finding is a linear correlation between the flow resistance of cement pastes and the specific surface of the cements multiplied with a weighed sum of cubic C(3)A and C3S, or in other words the surface fraction of the most hydraulic active minerals in the fresh state. Cements ground from the same clinker show an exponential relation between fineness and flow resistance of pastes. Flow resistance of pastes decreases with increasing volume replacement of silica fume until 9 vol% and increases again thereafter. Polyether grafted polyacrylates (PA) are better plasticizers than naphthalene sulphonate-formaldehyde condensate (SNF) and sodium lignosulphonate (LS). Unlike for SNF and LS, paste show no improved theology by delayed addition of PA.