Trichomes are specialized epidermal cells that are regularly distributed on the leaves of Arabidopsis plants. During leaf development, each trichome undergoes cell morphogenesis, resulting in a very precise architecture. We have isolated and characterized more than 70 trichome mutants that represent 21 different genes. By analyzing their cellular phenotypes, it has been possible to define specific steps in trichome development. Although these steps normally occur in a temporal sequence, genetic dissection indicates that the morphology of the mature cell is largely assembled in independent events. The results suggest a model of how a specialized cell type originates from a uniform cell layer.