Monocytes are agranular leukocytes that originate from hematopoietic stem cells (HSCs) in the bone marrow. They circulate in the peripheral blood and are recruited to tissues following injury or infection. Once in the tissue, monocytes differentiate into inflammatory macrophages, which play a role in initiating the pro-inflammatory immune response and eliminating infectious pathogens. In the absence of injury or infection, all tissues of the body also contain tissue-resident macrophages that are present throughout life. Tissue-resident macrophages have three possible origins: yolk sac-derived primitive macrophages, fetal liver-derived monocytes, or bone marrow-derived monocytes. Many tissue-resident macrophages with embryonic origins can self-renew and persist into adulthood, while those in other tissues are gradually replaced by bone marrow-derived monocytes. Tissue-resident macrophages have central roles in tissue development and homeostasis, immune surveillance, phagocytosis, antigen presentation, and wound healing. In addition to these common functions, tissue-resident macrophages can have specialized functions depending on the tissue in which they reside.