Publisher Summary This chapter discusses the early stages of eye development in Drosophila, focusing on the mechanisms that determine the global pattern rather than those involved in the establishment of specific cell fates. It discusses the specification of the eye disk to form an eye, the control of morphogenetic furrow initiation and propagation, and the coordination of growth with differentiation. Most adult Drosophila structures, including the eye, develop from imaginal disks. These are groups of cells set aside in the embryo that grow and differentiate inside the larva and evert to become functional during metamorphosis. Differentiation in the eye disk is progressive, moving across the disk in a wavelike manner from posterior to anterior. The front of the wave is marked by an indentation in the disk known as the morphogenetic furrow. Most cell division occurs in the unpatterned cells anterior to the furrow, while on the posterior side of the furrow the cells are organized into clusters that develop into the ommatidia. The Drosophila eye disk develops in a remarkably coordinated manner. Its differentiation begins at a precisely defined point and expands at a steady rate, laying down rows of evenly spaced ommatidial founder cells.