This phD thesis studies advice-giving and advice-taking in a situation of conflicting interests between judges and advisors. The first part looks more specifically at vague advice and examines all possible interpretations advisors can make of this vagueness. Our main result shows that perceived benevolence positively influences advice-taking. The second part investigates the effects of advisors' benevolence and competence on judges' trust when they use advice and on advisors' actual trustworthiness when they give advice. We compare situations with and without conflict of interests. Our results show that people are not sensitive to conflict of interests for advice-taking: they only use dispositional cues concerning the advisor. By contrast, they take into account the conflict when they give advice: the amount of help given to the judge depends on benevolence; the quality and sincerity of advice depend on advisors' competence. In the last part, we only study advice-giving in a situation of conflicting interests and we investigate more deeply the relation between advisors' competence and their sincerity. Our results show that uncertainty experienced by the least competent advisors about their advice can lead them to give unsincere advice because it less likely has a negative impact on judge. This phenomenon also occurs when uncertainty comes from the situation.