The institute of modification of public procurement contracts during their validity is exceptionally important for the field of public procurement, as the mere compliance with the regulation entailed in the public procurement legislation with regard to modifications of such contracts, with consideration of the case law, allows for a consistent realization of all founding principles based on which the objectives of the special regulation of public contracts are being realized, whereby such contracts must be dealt with separately from private contracts due to their specific nature. The Master's Thesis briefly describes the legal consequences of instances when the need for the modification of a specific public contract arises already prior to the conclusion of the said contract. The primary objective of the Master's Thesis, however, is a comprehensive overview of the regulation of the institute of modification of public procurement contracts during their validity, which especially entails the overview of the characteristics of public contracts, consideration and definition of the conditions that are stipulated for a permissible modification of contract by the public procurement legislation and the case law, as well as the consideration of the legal consequences of impermissible modifications of such contracts. An analysis of statistical data on the modifications of the said contracts in practice was carried out. The content also includes a comparison of the said matter with the general rules of contract law. This is followed by a description of a contracting authority's options to apply the negotiated procedure without prior publication which represents an additional legal basis for situations when the need arises for the modification of the initial public contract, and the possibility or duty of the contracting authority to, as a final consequence of certain instances, terminate a public procurement contract due to the circumstance of a substantially modificationd public contract. The Master's Thesis is concluded with final findings and proposals for the improvements of the field of this institute.