Arabidopsis thaliana is the model organism for the study of the higher plant circadian clock. The physiological change in position of young leaves and cotyledons in Arabidopsis seedlings reveals an overt circadian rhythm. Measuring these leaf movements provides a simple and reliable assay of the plant circadian clock and, unlike systems based on the firefly luciferase reporter gene, requires no prior genetic manipulation of the plant. As such, leaf movement can be used to measure circadian rhythms in plants lacking luciferase reporter genes, or as an independent measure of the clock in plants that do possess the transgene. The imaging system described in this chapter can also be adapted to measure circadian rhythms in other plant species displaying rhythmic leaf movements.