The interpreter who becomes the litigant's ears and mouth, must adhere to certain strict principles of behavior to insure an exact transmission of the content of the message in both directions so that all of the participants in the hearing or trial hear exactly what the witness and all of the others involved in the process present. This paper presents the fundamental principles that determine the conduct of the sworn translator in the state of Washington (USA), and which are summed up here: - Exact and complete interpretation of the message without adding or subtracting anything, and at the same time watching the language register. - Be careful not to fall into situations where the interpreter could have a conflict of interest. - Maintain absolute confidentiality. - Only intervene in situations where the interpreter is properly prepared. - Maintain a high level of personal and professional conduct.