The accumulation of lingering senescent cells in all tissues is one of the causes of aging. Even in very late life, senescent cells are thought to account for only a few percent at most of all cells in any given tissue, but they cause great disruption to tissue structure and function: chronic inflammation, impaired regeneration, fibrosis, and other unpleasant outcomes. This is accomplished via an as yet incompletely cataloged mix of secreted molecules known as the senescence-associated secretory phenotype, or SASP. Acting via secretions allows a small number of cells to have large effects. When present for a limited period of time, senescent cells are helpful, a necessary part of wound healing, embryonic development, and suppression of cancer. Cells become senescent in response to the circumstance