Macrophages are immune cells that are present in essentially every organ of the body. Besides classical functions in immunity, they are also important for normal functioning of the surrounding tissue and its regeneration. Resident macrophages can originate from embryonic progenitors and be maintained in tissues long term by local proliferation. This self-renewal ability, however, appears to decline with age, with potentially major consequences for the response to infection, the resolution of inflammation and the ability for tissue regeneration. Understanding the mechanisms of macrophage aging may thus hold great promise in diseases increasing with old age, including degenerative disease and cancer. We want to decipher the molecular and cellular parameters of macrophage self-renewal and aging. We believe that this understanding will lead to new treatments of such diseases based on manipulating macrophage activity states and macrophage cellular therapies.