Monocytes develop in the bone marrow and represent the primary type of mononuclear phagocyte found in the blood. They were long thought of as a source for tissue macrophages, but recent studies indicate more complex roles for monocytes, both within the circulation and after their migration into tissues and lymphoid organs. In this Review, we discuss the newer concepts underlying the maturation of emigrating monocytes into different classes of tissue macrophages, as well as their potential functions, as monocyte-derived cells, in the tissues. In addition, we consider the emerging roles for monocytes in adaptive immunity as antigen-presenting cells.