We review the various ways in which an electron beam can adversely affect an organic or inorganic sample during examination in an electron microscope. The effects considered are: heating, electrostatic charging, ionization damage (radiolysis), displacement damage, sputtering and hydrocarbon contamination. In each case, strategies to minimise the damage are identified. In the light of recent experimental evidence, we re-examine two common assumptions: that the amount of radiation damage is proportional to the electron dose and is independent of beam diameter; and that the extent of the damage is proportional to the amount of energy deposited in the specimen.