Unlike plants, which can continue to develop new organs and elaborate their body plan throughout their life, animals generally have to contend with a body plan which is fixed at birth. The basic body plan is laid out during embryogenesis, the intricately choreographed interplay of different processes and mechanisms involved in the development of the embryo. An important step early in animal embryogenesis is the lengthwise division of the embryo into a series of primitive segments, called somites; each somite will later give rise to vertebrae, skeletal muscle, and dermis, although some of the somites may fuse before this happens. Correct formation of the somites is a crucial component of the development and is precisely regulated in both space and time. This tight regulation is accomplished by a remarkably simple, beautiful and ancient mechanism.