The aim of this paper is to show the problem-solving activity of three Norwegian expert translators facing translation problems due to difficulties in understanding and/or reformulating the sense expressed in an utterance extracted from a French original text published in L’Express in January 2001. Two on-line methods involving observations of the expert translators’ behaviour generating process data have been combined: Think-aloud protocols and keyboard logging of the translators’ writing activities. The study shows which kind of knowledge is processed by the expert-translators in order to solve the problems at hand. It also appears from the study that there is a strong link between the problem-solving methods applied by the translator and the underlying translation principles.