Ultra-high-performance concrete (UHPC) with adapted rheology continues to attract interest considering the requirement for novel processing techniques such as self-consolidating, pumping, spraying, and three-dimensional (3D) printing. The rheology of UHPC is complex due to its high solid volume fraction, low water content, and wide range of constituent materials that affect its flow properties. This work provides guidance for tailoring the mixture proportioning of UHPC to secure proper rheological properties and performance of UHPC for various applications. In the first part of this work, key physical, physicochemical, and chemical factors that can affect the rheological properties of UHPC are discussed. Rheological measurement methods and interpretation of the test results are provided to accurately determine the rheological parameters. The effects of constituent materials on the yield stress, viscosity, thixotropy, and structural build-up of UHPC are elaborated. The rheological parameters can increase by up to 100 times with the decrease in water-to-binder ratio. Such an increase can be reduced to less than 10 times through optimization of the particle size distribution and selection of superplasticizer. Rheology control strategies for UHPC for various applications are outlined. Multiple chemical admixtures with an organized molecular architecture must be used to achieve contradictory rheological requirements (e.g., low yield stress but high viscosity; low dynamic yield stress but high static yield stress). Finally, challenges and future demands to fine-tune the rheological properties of sustainable UHPC are showcased. Of special interest in future studies is the interaction between low-clinker binder and chemical admixtures and its effect on the microstructure of fresh UHPC.