Against moral philosophers' traditional preoccupation with ‘ought’ judgments, Bernard Williams has reminded us of the importance of locutions such as ‘I must’, ‘I have to’ and ‘I can't’. He develops an account of the ethical necessity and impossibility these locutions are able to mark. The account draws on his thesis that all reasons for action are ‘internal’. I sketch the account, and then try to show that it is insensitive to important aspects of how the concepts of ethical necessity and impossibility inform our lives.