At the beginning of the 18th century, based on decisions made in 1699, the Cossack units, the so-called registered ones, which had earlier supported the state army in the defence of the south-eastern provinces of the Polish-Lithuanian Commonwealth for over 150 years, were liquidated. Those experiences related to numerous Cossack rebellions, the last in 1701–1703, meant that until 1792 it was not decided to recreate units recruited from Cossacks in the state army. However, the magnates and gentry were willing to use such units, by taking advantage of the applicable law, and they decided to maintain a court army. In the south-eastern provinces, having private militias was necessary in the face of raids by haidamaks (native bandits), Moldavian bandits, and Cossacks from the territories of Russia. Cossack units were easy to form due to a large recruitment base and cheap to maintain. The latter was due to granting farms exempt from all feudal services except compulsory police and military service. It is no wonder that Cossack units were an important, sometimes dominant element of the court armies of such large landowners as Chodkiewicz, Jabłonowski, Lubomirski, Potocki, or Radziwiłł (but with the reservation that only on their estates in Ukraine) presented in the article. The total number of court Cossack units was certainly much smaller than the 30,000 given by Kitowicz. However, they still constituted a significant group of combatants in the service of the magnates and the borderland nobility, protecting but also threatening their estates.