According to a classical view, secession is a purely factual phenomenon not regulated by international law. This thesis challenges this view. It demonstrates that international law is relevant to secession and establishes the extent of such relevance. The thesis is divided into two parts. The first part outlines a general international legal framework of secession. It demonstrates that violation of peremptory norms during secessionist attempt precludes the emergence of a new State and orders subsequent relations of the secessionist entity. The thesis coins the term an illegal secessionist entity and establishes an applicable legal framework, which includes the consequences of a change of territorial control and consequences of peremptory territorial illegality. The second part analyses case studies from the post-Soviet space, namely Nagorno-Karabakh, Transnistria, Abkhazia and South Ossetia, Crimea and Donetsk and Luhansk People’s Republics. It demonstrates that apart from Crimea, which is annexed by Russia, these entities are not States, but illegal secessionist entities. It establishes the applicability of law of occupation and European Convention on Human Rights in these contexts. In light of the duty of non-recognition, it examines the inter- State relations of these entities as well as their economic dealings and official acts and laws. The thesis demonstrates that effects of peremptory norms in the context of secession are rather expansive, thereby highlighting the role of international public order in contemporary regulation of secession.