Federal Rule of Evidence 601 provides that every witness is competent 'except as otherwise provided' in the Federal Rules of Evidence. The proviso refers in part to general competency requirements specified in Federal Rules of Evidence 602 and 603. Rule 602 requires the witness to possess personal knowledge. Rule 603 requires every witness to declare that he/she will testify truthfully by oath or affirmation. Together these rules require that the witness (1) have the capacity to accurately perceive, record, and recollect impressions of facts (physical and mental capacity); (2) in fact did perceive, record, and can recollect impressions having any tendency to establish a fact of consequence in the litigation (personal knowledge); (3) be capable of understanding the obligation to tell the truth (oath or affirmation); and (4) have the capacity to express himself/herself clearly with aid of an interpreter when necessary (narration). When a witness' acquisition of personal knowledge has been questioned, the court may reject the testimony only when no reasonable juror could believe that the witness perceived what he/she claims to have perceived. When the mental capacity of a witness has been questioned, the ultimate issue is whether the witness is so bereft of powers of observation, recordation, recollection, and narration as to be so untrustworthy a witness that the testimony lacks relevancy. Considering mental capacity and acquisition of personal knowledge together, the test of competency may be characterized as requiring minimum credibility. Judges increasingly tend to resolve all doubts about the minimum credibility of a witness in favor of permitting the jury to hear the testimony and judge the credibility of the witness for itself. Forty-two footnotes are provided.