The monograph is dedicated to the socio-political history of the Grand Duchy of Lithuania in 1430–1440. The dramatic events of this period - the rise to power of Svidrigail, his conflict with Poland, the overthrow and enthronement of Sigismund Keistutovich, his attempts to strengthen his power and death at the hands of the conspirators - are examined against the broad background of the social history of the largest state in Eastern Europe and its relations with its neighbors. The research is based on a wide range of sources, many of which are being introduced into scientific circulation.