The involvement of key factors operating independently or in cooperation with others contributes to physical and physiological mechanisms to help engineer a vertebrate hypothalamus. The actions of these key factors influence developmental mechanisms including neurogenesis, cell migration, cell differentiation, cell death, axon guidance, and synaptogenesis. On a molecular level, there are several ways to categorize the actions of factors that drive brain development. These range from the actions of transcription factors in cell nuclei that regulate the expression of developmental genes, to external factors in the cellular environment that mediate interactions and cell placements, and to effector molecules that contribute to signaling from one cell to another. Sexual dimorphism is a hallmark of the vertebrate hypothalamus that may arise as a direct consequence of hormone actions or gene actions. These actions may work through any of the mechanisms outlined above. Given the arrangement of cells in groups within the hypothalamus, cell migration may be one particularly important target for early molecular actions that help build the bases for appropriate functions.