The top aide to a Senator or Representative. The chief of staff is responsible for managing the Member"s entire staff and also acts as the Member"s chief political strategist. He/she is viewed as the Member"s alter ego and often attends meetings on behalf of the Member and speaks for him or her.
The position of Chief of Staff in the United Kingdom is an unelected position, with the post being chosen by the Prime Minister. He/She is listed as having 'direct responsibility for leading and co-ordinating operations across Number 10' and reports directly to the Prime Minister.