One of the most challenging problems in developmental biology is to identify the mechanisms that generate the complex arrangement and interconnections of cells in the vertebrate brain. This review surveys the evidence that the hindbrain region is laid down as a series of similar modules, final complexity being founded on this simple ground plan. Morphological studies, using immunohistochemical and axon-labelling techniques, reveal repeat patterns of neuronal differentiation and organization, whereas cell marking experiments have shown that the segments, or rhombomeres, are lineage-restriction units each constructing a defined piece of the hindbrain.