Change of epithelial cell fate occurs in developmental, physiological, and pathological processes. While the initiation and outcome of the various processes may differ, many of the cellular and molecular controls are common. Using Drosophila gastrulation as a model, we describe in this chapter the molecular genetics of cell shape changes of the epithelial cells in the blastoderm. The columnar epithelial cells in the ventral region, which are destined to become mesodermal precursors, behave differently from cells in other regions of the early embryo and invaginate to the appropriate position during gastrulation. Genetic studies point to the involvement of the region specific transcription factor Snail and the ubiquitous adherens junctions and cytoskeleton. How cell fate determinants turn on ubiquitous machineries to achieve the coordinated cell movements during gastrulation is being elucidated and is a fascinating area of research that attracts the attention of scientists from various disciplines.