Ageing is accompanied by a progressive decline in stem cell function, resulting in less effective tissue homeostasis and repair. In this unit lectures cover age-related stem cell dysfunction and various paradigms of how stem cell functionality changes with age, including impaired self-renewal and aberrant differentiation potential. As multicellular organisms age, there is a gradual loss of tissue homeostasis and organ function. Throughout life, populations of adult stem cells maintain many tissues, such as the blood, skin and intestinal epithelium. Therefore, it is likely that the decrease in tissue homeostasis can be attributed to an age-related decline in the ability of stem cells to replace damaged cells. Although cell autonomous changes occur as the organism ages that result in the inability of stem cells to proliferate or self-renew, or of daughter cells to differentiate along a specific lineage, local and systemic changes can also affect the ability of stem and progenitor cells to function properly.