 This study examined the effect of lightweight aggregates on the mechanical and microstructural properties of ultra-high-performance concrete, UHPC. The study found that increasing the amount of lightweight aggregates in the mix decreased the density and mass loss of the material, but also decreased the compressive and flexural strengths. Additionally, the study showed that the addition of lightweight aggregates increased the porosity of the material, but this increase was limited. Finally, the study found that the inclusion of lightweight aggregates had no significant effect on the microstructure of the material. This article was authored by RFAA Battle.