A witness is a third party outside the dispute between the parties, who provides specific information as testimonial evidence. In this polysemic expression, with a shared nature and complementary evidentiary effects, there is, on the one hand, the so-called technical witness, connected to the case, relevant for his factual presence and his knowledge or mastery of knowledge related to the facts previous to the event. On the other hand, there is the expert witness, identified without a testimonial nature of his own, but who acquires this denomination when he intervenes to give explanations of his opinion through cross-examination. In another particular area, the participation of the expert is evidenced, but without the capacity of technical witness or expert witness, who provides his expertise to assist in the evaluation of evidence. This expert has been named a new means of evidence by the Supreme Court of Justice of Colombia. This article characterizes the evidentiary contribution of each of these experts in their procedural intervention, from the model of tort liability for medical error. Then the levels of conviction are outlined and analyzed, in the variables of plus, superior and medium or inferior, on which the judge can place himself to justify the decision argumentatively from the contribution of expert information to the process. © 2023, Universidad Catolica de Colombia. All rights reserved.