My master thesis presents a comprehensive analysis of gap filling in contractual unified law, whereby I define unified contract law as the uniform rules applicable to international sales contracts, taking into account different social, economical and legal systems. The purpose of the unified law is to remove obstacles in international trade and thus strenghten its development. Accordingly, it is necessary to fill in the gaps in contractual unified law in a special way. After a brief explanation of what constitues gap filling in international law, I firstly focused on determining whether a gap even exists, otherwise the substance is already regulated by the Convention (UN Convention on International Sale of Goods; Convention on International Sale). Since the first step is very difficult, I critically assessed some of the methods described. Using a method incorrectly can not be relied upon to conclude that we should fill in a gap that may be imaginary. I further described the possible ways of gap filling. Those other method are governed by the hierarchy, which I followed and began by gap filling by analogy. A detailed overview of the most relevant general principles of law on which the Convention is based follows. Conditionally, we may also use external general principles of law in order to fill in the gaps, which I further described and demonstrated their nature. At this point I also explained the concept of lex mercatoria. I prepared a comparative legal analysis of the application of the UN Convention on International Sale of Goods, UNIDROIT – Principles of International Contract Law and the Principles of European Contract Law, and concluded with the rules deriving from private international law as the last method to be used for gap filling. My master's thesis is based on a theoretical basis filled with many practical examples and imaginary situations to provide a complete overview.