The vertebrate head is often regarded as a discrete developmental unit, distinct from the trunk and governed by a different patterning mechanism. The head is patterned according to the branchial arches with patterning information primarily derived from the endoderm, a principle called “branchiomerism”. The trunk in contrast is patterned according to the somites, segmented blocks of mesoderm, a principle called “somitomerism”. The region of the vertebrate body where these two distinct patterning mechanisms meet is termed the head/trunk interface. The co-existence of two fundamentally different developmental patterning principles at the same axial level of the body is unique to vertebrates. As far as we know, the body architecture of all other animals is always based on one underlying patterning principle. The vertebrate head/trunk interface therefore offers a unique model to study how two different patterning principles interact in development and if this interaction might have played a special role in the evolutionary diversification of vertebrates.