The use of the computer in translating natural languages ranges from that of a translator''s aid for word processing and dictionary lookup to that of a full-fledged translator on its own. However the obstacles to translating by means of the computer are primarily linguistic. To overcome them it is necessary to resolve the ambiguities that pervade a natural language when words and sentences are viewed in isolation. The problem then is to formalize, in the computer, these aspects of natural language understanding. The authors show how, from a linguistic point of view, one may form some idea of what goes on inside a system''s black box, given only the input (original text) and the raw output (translated text before post-editing). Many examples of English/French translation are used to illustrate the principles involved.