The monograph uncovers political and diplomatic aspects in the life of Michał Kleofas Ogiński (1765-1833), politician and diplomat of the Grand Duchy of Lithuania and Polish-Lithuanian state, in the wide context of interstate relations in East-Central Europe. Using plentiful archival materials the research focuses on objectives and results of the diplomatic missions to The Hague and London, analyses reform projects of the Polish-Lithuanian state and tackles the problem of high treason. The 1786-1794 section of Michał Kleofas Ogiński’s biography is supplemented with a publication of his travel documents.