The article consists in the review of the protection principie and its manifestation in the in dubio pro operario, from a foreign, especially continental European law, and Chilean law perspective. It is possible to conclude that the in dubio pro operario is nota principie ofLabor Law but only an interpretation criteria. The in dubio pro operario is not considered by the current European doctrine and jurisprudence. Even if estimated possible it should be applied in a very limited way. Its application could create legal uncertainty. It could never be used to fill a legal vacuum or to create situations not considered by the law.