This Paper Presents the Results of a Round-Robin Testing Program Undertaken by RILEM TC-266-Measuring Rheological Properties of Cement-Based Materials in May 2018 at the Université D'Artois in Bethune, France. Seven Types of Rheometers Were Compared; They Consisted of Four ICAR Rheometers, Viskomat XL Rheometer, EBT-V Rheometer, Sliding Pipe Rheometer (SLIPER), RheoCAD Rheometer, and 4SCC Rheometer, as Well as the Plate Test. This Paper Discusses the Results of the Evolution of the Static Yield Stress at Rest of Three Mortar and Five Concrete Mixtures that Were Determined using Two ICAR Rheometers, Viskomat XL, and EBT-V Rheometers, as Well as the Plate Test. for the Measurements Carried Out with Rheometers, Three Different Structural Build-Up Indices (I.e., Structural Build-Up Rate, Critical Time, and Coupled Effects of Initial Static Yield Stress and Rate of Structural Build-Up) Were Determined. the Indices Were Established using: (I) Two Static Yield Stress Values Measured after 10 and 40 Min of Rest; and (Ii) Two Static Yield Stress Values Measured after 10 and 40 Min of Rest Plus the Initial Dynamic Yield Stress (No Rest and Obtained from the Flow Curves). the Paper Discusses the Test Results and Highlights Inaccuracies that Could Be Encountered in Determining the Static Yield Stress. Test Results Indicate that the ICAR Rheometers and the Selected Thixotropic Indices Can Provide Similar Results, and that the Spread of Results Obtained from Different Rheometers Can Be Considerably Reduced When using Three Yield Stress Values to Calculate the Rate of the Static Yield Stress at Rest. in Order to Enhance the Accuracy of Measurements, It is Recommended to Increase the Number of Measurements of the Yield Stress to at Least Three Points over One Hour after Mixing.