Mononuclear phagocytes are crucial components of the innate host defense system. Cells such as macrophages and monocytes phagocytose and process pathogens, produce inflammatory mediators, and link the innate and the adaptive immune systems. The role of innate immune receptors such as Toll-like receptors (TLRs) in the recognition of pathogens is critical for mounting a precise and targeted immune response. This review focuses attention on the development of monocytes and macrophages, various populations of macrophages, and the expression and function of TLRs on macrophages.