Protection of trade mark is available for any sign which is capable of distinguishing the goods and services of one enterprise from others. These signs include, inter alia, words, personal names, letters and numerals, figurative signs, colors and color combinations, three dimensional shapes, including the shape of goods or their packaging. They are essential in market economies, fostering market transparency, permitting their owners to create a direct link with consumers, allowing consumers to identify and memorize the products and services they prefer, thus contributing to a system of fair and undistorted competition. However, with the growth of globalization, e-commerce consumers and advertising, reputation and distinctiveness of trade marks are particularly vulnerable to attack by those who wish to take advantage of them, for profits and enhance their financial gain (using a sign, identical or similar, to an earlier trade mark). The damage and exploitation of distinctiveness as well as the damage and the exploitation of the reputation of the well-known trademark can be discussed only in the event when there are no specific ‟due causes' coming from the side of a potential infringer. This means that the lack of ‟due cause' is the requirement for the protection of well-known trademarks. In this paper, we analyzed the BGH and the ECJ judgments whose subject was to determine the existence of ‟due causes'. Since this reason can be based on the regulations of trademark rights, but also on the basis of more general rules of legal order, such as constitutional provisions protecting freedom of thought, freedom of artistic creativity, as well as the communitarian regulations on freedom of transport and services in the single market, establishing a valid reason is based on all the circumstances of specific cases, which is why judicial decisions often come to opposite conclusion. ‟Due cause' limits the scope of protection of well known trademark. In terms of peacock sting, the limit should be reduced to the suitable measure, mainly in the sense that new competitors, with the use of additional features, must be distinguished from the already well known trademark.