Although the functional role of eosinophils is primarily considered to be host defense against parasitic infection, current studies indicate that this complex cell is ideally adapted for other roles that may involve immune modulation and tissue repair. The eosinophil is an important source of basic proteins, lipid mediators, cytokines, and growth factors. In disease states, eosinophil mobilization from the marrow and circulation can be very rapid yet highly organized and cell selective, particularly in response to interleukin (IL) 5 and eotaxin. Such elaborate recruitment is regulated by a series of interactions by eosinophil receptors and the endothelium and extracellular matrix ligands. Several priming mechanisms activate the eosinophils during this recruitment process so that the cell arrives at its tissue destination prepared for immediate action and prolonged tissue survival. Degranulation is strictly controlled and allows the cell to differentially release its contents in an ordered manner. This is essential to prevent injury to tissue during migration. Therapy is still limited for eosinophil-driven diseases, but intervention at the key events that govern eosinophil recruitment and effector function may be the way forward.