During an embryo’s journey from a single cell to a complex organism, countless patterning processes make sure that the right cells develop in exactly the right location and at the right time. Cells activate specific genes in a rhythmic manner during this early development, resulting in waves of activation sweeping through the embryo. Scientists at EMBL now show that the rhythm between two specific sets of waves—controlled by the Wnt and Notch pathways—enables the formation of new segments. They have reported the results of their study in Cell.