  We develop a Galois (descent) theory for comonads within the framework of
bicategories. We give generalizations of Beck's theorem and the Joyal-Tierney
theorem. Many examples are provided, including classical descent theory,
Hopf-Galois theory over Hopf algebras and Hopf algebroids, Galois theory for
corings and group-corings, and Morita-Takeuchi theory for corings. As an
application we construct a new type of comatrix corings based on (dual) quasi
bialgebras.
