The paper deals with the issue of negotiations in the context of megaprojects with reference to the importance of the good faith principle. After an introduction to the meaning of such a principle, the author attempts to identify both its content and consequences that may result from behaviour in bad faith in the case of negotiations functional to megaproject contracts. Firstly, it is discussed the scenario where good faith imposes obligations which do not necessarily relate to negotiations, but are only incidental to them, as might be about knowledge of confidential information. Secondly, it examines cases where good faith imposes obligations that are directly related to negotiations. The most relevant and problematic of these, even in megaprojects, is breaking off negotiations. A distinction is made between situations where the client is a private party and where it is a public authority. Finally, it considers the case where public administration is not the megaproject’s client, but it is involved in obtaining the authorisations necessary for works to start. Public administration’s conduct might give rise, indeed, to a legitimate expectation of such authorisation, which is ultimately not granted, even if lawfully at the level of administrative discretion, but which causes harm to private party.