  A new description of the dynamics of warped accretion discs is presented. A
theory of fully nonlinear, slowly varying bending waves is developed, involving
a proper treatment of viscous fluid dynamics but neglecting self-gravitation.
The special case of resonant wave propagation found in inviscid Keplerian discs
is not considered. The resulting description goes beyond existing linear and
nonlinear theories, and is suitable for numerical implementation.
