The work deals with the molecular mechanisms of the development of resistance of HIV to modern preparations for treatment of HIV infection and AIDS: inhibitors of reverse transcriptase and protease. The principles and concrete schemes of the use of modern methods of geno- and phenotypic resistance determination of HIV are described in detail. The comparative characterization of both approaches together with the problems and difficulties connected with the interpretation of the results of resistance tests are presented, their probable clinical importance is discussed. Practical recommendations concerning the use of tests for the determination of HIV resistance under different clinical conditions, based on the conferences and literature data on HIV infection and for the period of 1999-2000, are reviewed.