Cell migration is the directed movement of a single cell or a group of cells in response to chemical and/or mechanical signals. It is a fundamental cellular process that occurs throughout life, starting during embryonic development and continuing until death, and at times it can contribute to pathogenic states in disease. In a developing embryo, cell migration is the driving factor for various morphogenetic events. For instance, during gastrulation in very early embryos, groups of cells migrate as sheets to form the three germ layers. Subsequently, cells from the germ layers migrate to various target locations, where they specialize into distinct cell populations that make up various tissues or organs in the embryo. In adult organisms, cell migration occurs during vital cellular processes such as tissue renewal and repair, wherein old or damaged cells are replaced by the migration of newly formed cells from the underlying tissue layers. Such events are essential to maintain tissue integrity and homeostasis. Cell migration also plays a role in mediating immune responses during infections, in which phagocytic cells such as neutrophils circulating in the bloodstream migrate to the infected tissues and destroy the invading pathogens.