The rhizosphere is a key interface between plants, microbes and the soil which influences plant health and nutrition and modulates terrestrial biogeochemical cycling processes. Recently it has been shown that the rhizosphere environment is far more dynamic than previously recognized, with evidence emerging for diurnal rhythmicity in rhizosphere chemistry and in the composition of rhizosphere microbial communities. Rhythmicity of the rhizosphere microbiome is in part linked to the host plant's circadian rhythm, although evidence also points to some heterotrophic rhizosphere bacteria and fungi possessing rhythmicity. We review the evidence for diurnal rhythmicity in the composition and abundance of rhizosphere microbial communities, and its link to the plant circadian clock. Factors which may drive microbial rhythmicity in the rhizosphere are discussed, including diurnal change in the flux and composition of root exudates, rhizosphere physico-chemical properties and plant immunity mechanisms. The potential contributions of microbial processes to community rhythmicity are considered, including self-sustained microbial rhythms, bacterial movement into and out of the rhizosphere, and microbe-microbe interactions. We also consider evidence that changes in microbial composition mediated by the plant circadian clock may affect microbial function and its significance for plant health and broader soil biogeochemical cycling processes. We identify key knowledge gaps and approaches which could help to resolve the spatial and temporal variation and functional significance of rhizosphere microbial rhythmicity. This includes unravelling the factors which determine the oscillation of microbial activity, growth and death, and cross-talk with the host over diurnal time frames. We conclude that diurnal rhythmicity is an inherent characteristic of the rhizosphere environment and that temporal factors should be considered and reported in experimental studies conducted on the rhizosphere.