The purpose of this paper is to give complex and detailed overview on the application of the Principles of European Private Law (hereafter only 'PECL'). At the beginning the article describes in detail the formation of PECL in context with other initiatives aiming to create parts of European private law, and introduces the content of its three parts. The core of the contribution focuses on legal nature and possibilities of their use in practice. Taking into consideration the PECL do not have binding legal nature, the question arises whether they can be perceived as 'law' or the source of law. In this regard, two different views have occurred. First, traditional positivist view, claims that the PECL can not serve as the source of law because they lack the state authority, whereas the state is the only legal body entitled to create binding legal rules. Second, more liberal view supported mainly by professor Jansen, argues that the party autonomy is the core of any contract law and the state is not the only authority empowered to create the rules of law. Because the PECL have a facultative character, that can be used when chosen by the parties in two different ways; fist, as a material choice of law, and second, as a collision choice of law. The authors analyse this issue in connection with the Slovak Act No. 97/1963 Coll. on international private and procedural law, and also in the light of the new Regulation No. 593/2008 of the European Parliament and of Council of 17 June 2008 on the law applicable to contractual obligations. It analyses its articles and problematic formulation of the preamble. In the final part of the contribution the authors deal with limitations of the application of PECL contained in its article 1:103 requiring observance of mandatory rules, thereby concluding the complex analysis on the use of PECL in practice.