Chronic inflammation represents a long-term reaction to an inflammatory stimulus characterized by continued recruitment of mononuclear leukocytes (monocytes and lymphocytes) accompanied by tissue injury due to the sustained inflammatory response. In contrast to acute inflammatory responses, chronic inflammation may last weeks, months, or even a lifetime in the case of some chronic inflammatory diseases. In addition to the accumulation of monocyte-derived macrophages and lymphocytes, chronic inflammation is characterized by changes associated with wound healing, such as proliferation of fibroblasts and small blood vessels. Many chronic inflammatory diseases begin as low-grade, protracted responses to pathogens or certain endogenous or exogenous substances. Chronic inflammation plays a key role in the development and progression of many chronic diseases including, but not limited to, autoimmune diseases, metabolic disorders such as atherosclerosis and obesity, fibrosis, and cancer. Macrophages are key cellular elements of chronic inflammatory responses. Distinct populations of M1 and M2 macrophages regulate the chronic inflammatory environment. Some chronic inflammatory responses may proceed to granulomatous inflammation, characterized by a morphologically distinct cellular attempt to contain an offending agent or substance that is difficult to eradicate. The classical example of granulomatous inflammation is observed in the host response to infection with Mycobacterium tuberculosis.