In the work presented in this thesis, the rheology of cement pastes and fresh mortars was studied as a step towards understanding the rheological principles underlying the vibration and the compaction of concrete. For this purpose, an accurately machined double-gap rotational viscometer, having a thermocouple junction embedded in the wall of its middle cylinder, was especially constructed. The thermocouple was used to measure the rise in the temperature of the material under test. Neat cement pastes and cement-sand mortars of various consistencies were tested in the viscometer under either constant or variable rates of shear, and their flow characteristics investigated. The effect of shear on their yield value was also studied. From the results of a number of such tests, it was established that the shear stress induced in the viscometer gaps produces a significant reduction in the plastic viscosity of the cement paste matrix. It is suggested that, subject to a certain assumption being made, this breakdown can be classified as thixotropic. The effect of various factors on the energy input to the paste was also investigated and a method for measuring the workability of cement pastes and mortars suggested. A theoretical approach to the breakdown of the paste is presented. In the light of the conclusions drawn concerning the behaviour of the cement paste, some doubts are expressed regarding the current method of measuring the concrete workability by the Vebe apparatus, and an attempt is made to explain the rheological aspects of concrete vibration.