The Fall Workshops on Geometry and Physics have been held yearly since 1992, and bring together Spanish and Portuguese geometers and physicists, along with an ever increasing number of participants from outside the Iberian peninsula. The meetings aim to provide a forum for the exchange of ideas between researchers of different fields in Differential Geometry, Applied Mathematics and Physics, and always include a substantial number of enthusiastic young researchers amongst the participants. In October 1992 a meeting organized by the Instituto de Matemáticas y Física Fundamental of Consejo Superior de Investigaciones Científicas (Manuel de León) and the Departamento de Física Teórica II of the Universidad Complutense (Alberto Ibort) was held in Madrid. This workshop gave birth to an annual sequence of meetings, originally named Differential Geometry and its Applications (WODGA). Later, this name turned into the current one. This year the XXIth Workshop will take place in Burgos University from August 30 until September 1. Note that the closely related Third Iberoamerican Meeting on Geometry, Mechanics and Control will be held in Salamanca from September 3 to September 7. MAIN TOPICS The Workshop is open to any topic in the interplay between Geometry and Physics, including -Lie algebroids and mechanics -Lorentz geometry -Poisson geometry -Riemannian and pseudo-riemannian geometry -Symplectic and contact geometry -Integrable systems -Control theory -Classical theory of fields -Mechanics of continuous media -Quantum mechanics -Relativity -Supergravity and supersymmetry -String theory -Quantum gravity INVITED SPEAKERS: -Henrique Bursztyn (IMPA, Rio de Janeiro). -Marisa Fernández (U. País Vasco). -Mark Gotay (U. British Columbia). -Catherine Meusburger (FAU Erlangen-Nürnberg). -Yolanda Lozano (U. Oviedo). -Chris Rogers (GAU Göttingen).