Grasslands on the calcareous soils of chalk and other limestones are among the most species-rich plant communities in Europe. Huge losses among these grasslands and their continuing vulnerability to agricultural improvement, neglect, and the impact of climate change and pollutants have focused attention on the need for conservation of their biodiversity. A clear understanding of the molecular mechanisms that enable calcicole species to thrive on calcareous soils is essential to enable us to predict how these plant communities and their constituent species will be affected by environmental change and how the biodiversity of these ecosystems can be sustained. We have tested the hypothesis that adaptation to a calcareous environment reflects altered patterns of gene expression using Arabidospsis thaliana as a model. This species occurs widely throughout the U.K. in a range of habitats, both natural and artificial, that are varyingly calcareous. We have used transcript profiling by microarray analysis to identify genes that are differentially expressed between calcicolous and non-calcicolous A. thaliana accessions grown at high and low rhizospheric calcium and have used T-DNA knockouts to test their role in the calcicole phenotype. In addition, we have used a similar genomics approach to examine whether these calcicole adaptation genes show similar patterns of expression in wild A. thaliana and related species.