The picture of the physical nature of cometary nuclei has been changing dramatically since Giotto's encounter with 1P/Halley. Physical strength is one of the parameters that are very poorly known and that seem to experience a revision with every encounter with a comet-like object. This talk tries to summarise what we know and what we don't know about the physical strength of comet nucleus surface material. It uses information from laboratory experiments, computer models and in-situ measurements from the Rosetta Philae lander.