In this paper I will try to show that the Kasimov khanate was founded in 1445 as a result of an oral agreement between the exiled khan of the Golden Horde Ulug-Muhammad and Muscovite Grand Prince Vasilii II. Its foundation wasn't a voluntary measure of Muscovite rulers. It was founded in close connection with the relations between Rus' and the Golden Horde in general. It could arise only in the atmosphere of hostility and implacability typical for the Muscovite Grand Prince's house under the conditions of the rather frequent change of the Grand Princes. To provide you some historical basis, I will first speak briefly about the historiography of the problem and present my own view based mainly on the data of the “dogovornye gramoty” (treaties between Russian Princes) and “letopisi” (chronicles) afterwards.