Many plant species bear thorns, yet little is known as to how they develop. Thorns are thought to have evolved as a protection mechanism against predators. The distribution of thorn bearing species across angiosperm phylogeny suggests that thorns have evolved many times and represent an example of convergent evolution. There are many examples of species bearing thorns including the climbing ylang-ylang (Magnoliales, Annonaceae), gorse (Fabales, Fabaceae), hawthorn (Rosaceae) and Natal plum (Gentianales, Apocynaceae) and Citrus (Sapindales, Rutaceae). We are using the Rutaceae as a model group, to understand what are the molecular mechanisms required for thorn formation and whether or not similar processes were used during evolution. This group is ideal to explore these questions as there have been several gains and losses of thorn development in this family. In addition the genomes of two Citrus species have been sequenced, which facilitate molecular approaches. To assess the molecular mechanisms controlling thorn formation in Citrus, closely related species and cultivars are being scored for thorniness and representative species with and without thorns identified to determine polymorphism and relative expression of key genes hypothesised to be involved in thorn development. In addition, experiments are underway to determine the molecular basis for thorn production in Citrus.