2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/12982A 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.10 pages, 4 figures, to be published in Proceedings of the EC Summer School on Astrophysical Discs, eds J. A. Sellwood and J. GoodmanAstrophysicstext