The chapter attempts to identify the most important issues relating to illicit firearms trade in Eastern Europe. Phenomena such as drug-related crime, human trafficking, money laundering, and illicit firearms trade were new to this part of Europe, expanding in the 1990s. The mechanisms of arms trafficking in Eastern Europe in the 1990s were based on fierce competition in the criminal world, which rapidly underwent the processes of professionalization and brutalization. Organized criminal groups created a demand for weapons and ensured supply by organizing smuggling, among other activities. Nonetheless, illicit firearms trafficking is not the main focus of organized criminal groups because the trade in illicit firearms does not generate high income, especially when compared to drug or human trafficking. The scarce available data allowed us to conclude that organized criminal groups have firearms at their disposal but have not created illegal arsenals. In Eastern Europe, excavation of World War I and World War II battlefields is a specific source of illicit weapons. The chapter also deals with the war in Ukraine, a by-product of which is a potentially large firearms black market. In the future, it may constitute a factor that will continue to destabilize public order and security in the region. It will be necessary to develop strategies to fight the illicit trade in firearms originating in Ukraine, with a particular focus on combatting the smuggling of illicit firearms.