The aim of the paper is to introduce and analyse the binary code of the main principle applied in legal assistance procedures in international criminal matters – the locus regit actum and the forum regit actum sub-principles are characterized by different dynamics and have different legal consequences. While locus regit actum requires the application of the procedural rules of the state that has been addressed with the legal assistance request, the forum regit actum principle asks for application of the requesting state’s rules on the given procedure. The result of the legal assistance procedure is the evidence assumed to be used in the criminal procedure to be carried out in the requesting state. The different concepts have very distinguished consequences in the scope of admissibility of the evidence and regarding the legal remedies against obtaining the evidence. Within the European Union, the related developments show a double paradigm shift in this regard – the initial followed locus regit actum was abandoned in 2000 in favour of the forum regit actum, but then in 2017, the member states of the EU opted once again for locus regit actum with the new regime of the European Investigation Order. Finally, the paper highlights the main issues of the concept of the free movement of evidence and shares in addition to positive evaluation, criticism on the subject as well.