This work addresses issues that may arise during the execution phase of public contracts, primarily those concerning supplies and services, through the lens of Article 129 of Law 4412/2016, with the supplementary application of the Civil Code, as interpreted by the Author in his professional practice. Specifically, the analyzed issues clearly include cases of exemption of the contractor of a public supply or service contract, the consequences and mainly the rights of the contractor of the same contract when the Administration culpably fails to fulfill its own contractual obligations, the possibility or impossibility for the contracting authority to claim further compensation beyond the scope defined by the sanctioning measures in the procedure of declaring the contractor forfeited, as well as the possibility of terminating a public contract by mutual consent of the contracting parties.

As the provision of Article 129 of Law 4412/2016 appears to go beyond a limited application of the Civil Code rules, which was accepted both by theory and the established case law of administrative courts concerning public contracts, the study focuses on the application of the Civil Code provisions regarding the abnormal development of a public contract (inability to perform, default of debtor and creditor, defective performance), as well as the possibility of supplementary application of the Civil Code provisions concerning synallagmatic contracts, bearing in mind that the Civil Code rules are not applied in their entirety due to the public interest parameter. Starting from the position that private law does not constitute a foreign body within administrative law, and also the legislator's imperative – through Article 129 of Law 4412/2016 – for the supplementary application of the Civil Code to extend beyond those provisions expressing the general principle of good faith, this book aspires to contribute practical solutions through the examination of the aforementioned issues.