Experimental studies in chick and analysis of mouse mutants have provided a framework for studying the early developmental processes involved in specifying the cerebellar anlage. Fate mapping studies in chick have shown that at early stages the cerebellum derives from cells in the mesencephalon and metencephalon (mes-met). Transplantation studies in chick have implicated the mes-met junction (isthmus) as a source of secreted factors that organize development of the entire mes-met, perhaps by stimulating proliferation and specifying positional values across the region. Fgf-8 has been implicated as a major factor involved in the isthmus organizing activity. Gene expression studies indicate that the anterior and posterior expression domains of the homeobox genes Otx-2 and Gbx-2, respectively, are the earliest indication of a division of the brain. Furthermore, the Otx-2/Gbx-2 expression border later resides at the mes-met junction. Genetic studies in mouse have shown that Otx-2 and Gbx-2 are required for normal development of cells on both sides of the border. In addition, mutations affecting the secreted factor Wnt-1, which is expressed anterior to the Otx-2/Gbx-2 expression border and the homeodomain transcription factors, Engrailed-1,2 and Pax-2,5 that have broad overlapping expression domains in the mes-met, result in deletions of mes-met structures. Taken together, these studies suggest that specification of the cerebellar territory requires a hierarchy of complex cellular and genetic interactions that gradually subdivide the brain into smaller regions.