April 2,2021 marks the 25th anniversary of the signing of the Treaty on the Union between Belarus and Russia, which laid the foundation for integration of the two countries. The article analyses results of the twenty- five-year development of the Russia-Belarus relations and assesses their pros- pects with account taken of the events developing in Belarus in 2020–2021. Considering the activities of the Union State of Russia and Belarus, the author summarises two key historical periods in the association functioning and gives their analytical characteristics. The paper focuses on the main integration achievements of cooperation in spheres of the economy (interstate trade, mutual investments, joint ventures, industrial and high tech cooperation, cus- toms policy), defence (organising a common defence space, mutual armament supplies, manoeuvres), science and culture. The author also studies numerous issues that arise from time to time in the relations between the Russian Feder- ation and the Republic of Belarus (energy disputes, the Dairy and Meat Wars, etc.). Finally, the paper contains a detailed analysis of the development scena- rios for the Russia-Belarus relations on the immediate and mid-term horizons.