The animal and plant kingdoms use many of the same molecular tools to build decidedly different multicellular organisms. Learning how plants approach challenges common to both kingdoms can inspire new ways of thinking in the animal biologist. This primer introduces how a weed from the mustard family, Arabidopsis thaliana, has been used to work through developmental problems. It also compares and contrasts gene regulation tools in animals and plants. Accompanying the primer is a discussion of current topics in root development with Arabidopsis researchers Philip N. Benfey, Ph.D., and Kenneth D. Birnbaum, Ph.D. Developmental Dynamics 238:2449–2458, 2009. © 2009 Wiley-Liss, Inc.