Genomics is the discovery and study of many genes simultaneously on a genome - wide scale. The completion of the genome sequence of Arabidopsis thaliana (a model dicot) heralded the beginning of the genome era for plant biology. The development of genomic tools, such as microarray technologies are profoundly changing and accelerating research in many areas of biology including plant biology. DNA microarrays consist of thousands of target CDNAs robotically arrayed on glass slides. Flourescently labelled CDNA samples, from different tissues or different conditions, are then hybridised to the arrays. By analysing the fluorescence of the hybridised spots on the microarrays we can assess the gene expression changes of 1000's of genes simultaneously. Microarrays provide a powerful tool for discovery of plant genes involved in important biological processes such as growth, development and defence, to name but a few. Genomics-based characterization of plant genomes has the potential to revolutionize plant science.