This thesis aims at finding if the constitutional law of Germany, France and the United- Kingdom allows the citizens of these countries to access information held by public authorities, and to compare the legal mechanisms involved. The question of the existence of a general right of access to information held by public authorities is well known in the field of administrative law. The French (17 July 1978), German (5 September 2005) and British (30 November 2000) legislators have answered to this question. On the contrary, the question of the conformity of these legislative texts (with the exception of the British act) to the local constitutional law has been largely left unanswered. Indeed, a possibility exists that those laws are unconstitutional, be it by creating a too wide or too narrow right of access. Firstly, the German Federal Constitutional Court never clearly stated the existence or non-existence of a constitutional right of access to administrative documents that could be invoked against the legislator. Secondly, the French Constitutional Council never had a say as to the constitutionality of the 1978 law or the recent Code Regarding the Relations Between the Public and the Administration. Thirdly, the British case law is still divided regarding the relationships between the various sources of law on the matter, so that the exact relationship between the Freedom of Information Act 2000, common law and the royal prerogative remains unclear. A comparative analysis allows for a better understanding of the constitutional law regarding access to the information held by public authorities. Through that lens, it appears that in Germany, it is impossible to invoke a general right of access to such information through relying on the words of the Fundamental Law. The legislator is left free. In France, on the contrary, such a right exists within the article 15 of the French Declaration of the Rights of Man and of the Citizen, so that the constitutionality of the laws on the matter could be challenged through individual constitutional litigation. In the United-Kingdom, the said disagreements regarding the ranks and validity of prerogative powers and common law do not weigh on the validity of the Freedom of Information Act 2000, which can be considered as the sole constitutional source of public information law. More precisely, the constitutional laws of the compared states present the following situations: a delegation to the legislator of the power to create or not to create a general right of access to information held by public authorities (Germany), a general but highly imprecise right of access (France) and a non-general but highly precisely stated right of access (United-Kingdom).