Review of the hardback: 'This book is a beautiful presentation of Janelidze's general categorical Galois theory ... a rewarding read.' Laszlo Marki, Acta Sci. Math. Review of the hardback: '... highly recommended or anyone wishing to learn the mathematical side of category theory (rather than its computer-science aspect) ... I enjoyed reading it very much.' Proceedings of the Edinburgh Mathematical Society Review of the hardback: 'A comprehensive account, which may well deepen one's understanding of the classical case.' Mathematika