Cellular senescence is a key process that limits cancer development. The basics of our current understanding of the process are presented, including the genetic and epigenetic events that lead to the senescent phenotype. In addition to limiting replicative lifespan, senescence appears to act as a potent signal to activate immune clearance and alter the microenvironment. Finally, senescence induction is an area for anti-cancer therapeutic development, but will require the development of appropriate assays to identify the relevant target(s) in individual tumours. Copyright © 2013 Pathological Society of Great Britain and Ireland. Published by John Wiley & Sons, Ltd.